--- a/accessible/base/TreeWalker.cpp
+++ b/accessible/base/TreeWalker.cpp
@@ -340,12 +340,11 @@ TreeWalker::AccessibleFor(nsIContent* aN
   }
 
   return nullptr;
 }
 
 dom::AllChildrenIterator*
 TreeWalker::PopState()
 {
-  size_t length = mStateStack.Length();
-  mStateStack.RemoveElementAt(length - 1);
+  mStateStack.RemoveLastElement();
   return mStateStack.IsEmpty() ? nullptr : &mStateStack.LastElement();
 }

--- a/dom/animation/KeyframeUtils.cpp
+++ b/dom/animation/KeyframeUtils.cpp
@@ -1183,17 +1183,17 @@ HandleMissingFinalKeyframe(nsTArray<Anim
 
   // If the preference of the core Web Animations API is not enabled, don't fill
   // in the missing keyframe since the missing keyframe requires support for
   // additive animation which is guarded by this pref.
   if (!nsContentUtils::AnimationsAPICoreEnabled()) {
     // If we have already appended a new entry for the property so we have to
     // remove it.
     if (aCurrentAnimationProperty) {
-      aResult.RemoveElementAt(aResult.Length() - 1);
+      aResult.RemoveLastElement();
     }
     return;
   }
 
   // If |aCurrentAnimationProperty| is nullptr, that means this is the first
   // entry for the property, we have to append a new AnimationProperty for this
   // property.
   if (!aCurrentAnimationProperty) {

--- a/dom/base/TimeoutManager.cpp
+++ b/dom/base/TimeoutManager.cpp
@@ -151,17 +151,17 @@ TimeoutManager::CreateFiringId()
   return id;
 }
 
 void
 TimeoutManager::DestroyFiringId(uint32_t aFiringId)
 {
   MOZ_DIAGNOSTIC_ASSERT(!mFiringIdStack.IsEmpty());
   MOZ_DIAGNOSTIC_ASSERT(mFiringIdStack.LastElement() == aFiringId);
-  mFiringIdStack.RemoveElementAt(mFiringIdStack.Length() - 1);
+  mFiringIdStack.RemoveLastElement();
 }
 
 bool
 TimeoutManager::IsValidFiringId(uint32_t aFiringId) const
 {
   return !IsInvalidFiringId(aFiringId);
 }
 

--- a/dom/base/nsDocumentEncoder.cpp
+++ b/dom/base/nsDocumentEncoder.cpp
@@ -818,17 +818,17 @@ nsDocumentEncoder::SerializeRangeContext
   nsresult rv = NS_OK;
   for (nsINode* node : Reversed(serializedContext)) {
     rv = SerializeNodeEnd(node, aString);
 
     if (NS_FAILED(rv))
       break;
   }
 
-  mRangeContexts.RemoveElementAt(mRangeContexts.Length() - 1);
+  mRangeContexts.RemoveLastElement();
   return rv;
 }
 
 nsresult
 nsDocumentEncoder::SerializeRangeToString(nsRange *aRange,
                                           nsAString& aOutputString)
 {
   if (!aRange || aRange->Collapsed())

--- a/dom/base/nsFrameMessageManager.cpp
+++ b/dom/base/nsFrameMessageManager.cpp
@@ -1100,17 +1100,17 @@ nsFrameMessageManager::ReceiveMessage(ns
                                   JS::HandleValueArray(argv), &rval)) {
           continue;
         }
         if (aRetVal) {
           ErrorResult rv;
           StructuredCloneData* data = aRetVal->AppendElement();
           data->Write(cx, rval, rv);
           if (NS_WARN_IF(rv.Failed())) {
-            aRetVal->RemoveElementAt(aRetVal->Length() - 1);
+            aRetVal->RemoveLastElement();
             nsString msg = aMessage + NS_LITERAL_STRING(": message reply cannot be cloned. Are you trying to send an XPCOM object?");
 
             nsCOMPtr<nsIConsoleService> console(do_GetService(NS_CONSOLESERVICE_CONTRACTID));
             if (console) {
               nsCOMPtr<nsIScriptError> error(do_CreateInstance(NS_SCRIPTERROR_CONTRACTID));
               error->Init(msg, EmptyString(), EmptyString(),
                           0, 0, nsIScriptError::warningFlag, "chrome javascript",
                           false /* from private window */);

--- a/dom/base/nsHTMLContentSerializer.cpp
+++ b/dom/base/nsHTMLContentSerializer.cpp
@@ -323,17 +323,17 @@ nsHTMLContentSerializer::AppendElementEn
       return NS_OK;
     }
   }
   else if (mIsCopying && name == nsGkAtoms::ol && ns == kNameSpaceID_XHTML) {
     NS_ASSERTION((!mOLStateStack.IsEmpty()), "Cannot have an empty OL Stack");
     /* Though at this point we must always have an state to be deleted as all
     the OL opening tags are supposed to push an olState object to the stack*/
     if (!mOLStateStack.IsEmpty()) {
-      mOLStateStack.RemoveElementAt(mOLStateStack.Length() -1);
+      mOLStateStack.RemoveLastElement();
     }
   }
 
   if (ns == kNameSpaceID_XHTML) {
     bool isContainer =
       nsHTMLElement::IsContainer(nsHTMLTags::CaseSensitiveAtomTagToId(name));
     if (!isContainer) {
       // Keep this in sync with the cleanup at the end of this method.

--- a/dom/base/nsXHTMLContentSerializer.cpp
+++ b/dom/base/nsXHTMLContentSerializer.cpp
@@ -450,17 +450,17 @@ nsXHTMLContentSerializer::CheckElementEn
   aForceFormat = !(mFlags & nsIDocumentEncoder::OutputIgnoreMozDirty) &&
                  aElement->HasAttr(kNameSpaceID_None, nsGkAtoms::mozdirty);
 
   if (mIsCopying && aElement->IsHTMLElement(nsGkAtoms::ol)) {
     NS_ASSERTION((!mOLStateStack.IsEmpty()), "Cannot have an empty OL Stack");
     /* Though at this point we must always have an state to be deleted as all
        the OL opening tags are supposed to push an olState object to the stack*/
     if (!mOLStateStack.IsEmpty()) {
-        mOLStateStack.RemoveElementAt(mOLStateStack.Length() -1);
+      mOLStateStack.RemoveLastElement();
     }
   }
 
   bool dummyFormat;
   return nsXMLContentSerializer::CheckElementEnd(aElement, dummyFormat, aStr);
 }
 
 bool

--- a/dom/canvas/CanvasRenderingContext2D.cpp
+++ b/dom/canvas/CanvasRenderingContext2D.cpp
@@ -2234,17 +2234,17 @@ CanvasRenderingContext2D::Restore()
   }
 
   for (const auto& clipOrTransform : CurrentState().clipsAndTransforms) {
     if (clipOrTransform.IsClip()) {
       mTarget->PopClip();
     }
   }
 
-  mStyleStack.RemoveElementAt(mStyleStack.Length() - 1);
+  mStyleStack.RemoveLastElement();
 
   mTarget->SetTransform(CurrentState().transform);
 }
 
 //
 // transformations
 //
 

--- a/dom/media/webrtc/MediaTrackConstraints.cpp
+++ b/dom/media/webrtc/MediaTrackConstraints.cpp
@@ -526,17 +526,17 @@ MediaConstraintsHelper::SelectSettings(
         rejects.AppendElement(Move(aDevices[j]));
         aDevices.RemoveElementAt(j);
       } else {
         ++j;
       }
     }
     if (aDevices.IsEmpty()) {
       aDevices.AppendElements(Move(rejects));
-      aggregateConstraints.RemoveElementAt(aggregateConstraints.Length() - 1);
+      aggregateConstraints.RemoveLastElement();
     }
   }
   return nullptr;
 }
 
 /* static */ const char*
 MediaConstraintsHelper::FindBadConstraint(
     const NormalizedConstraints& aConstraints,

--- a/dom/workers/WorkerLoadInfo.cpp
+++ b/dom/workers/WorkerLoadInfo.cpp
@@ -524,17 +524,17 @@ InterfaceRequestor::GetAnyLiveTabChild()
 
     // Does this tab child still exist?  If so, return it.  We are done.  If the
     // PBrowser actor is no longer useful, don't bother returning this tab.
     if (tabChild && !static_cast<TabChild*>(tabChild.get())->IsDestroyed()) {
       return tabChild.forget();
     }
 
     // Otherwise remove the stale weak reference and check the next one
-    mTabChildList.RemoveElementAt(mTabChildList.Length() - 1);
+    mTabChildList.RemoveLastElement();
   }
 
   return nullptr;
 }
 
 NS_IMPL_ADDREF(WorkerLoadInfo::InterfaceRequestor)
 NS_IMPL_RELEASE(WorkerLoadInfo::InterfaceRequestor)
 NS_IMPL_QUERY_INTERFACE(WorkerLoadInfo::InterfaceRequestor,

--- a/dom/xbl/nsBindingManager.cpp
+++ b/dom/xbl/nsBindingManager.cpp
@@ -465,17 +465,17 @@ nsBindingManager::ExecuteDetachedHandler
 
   nsCOMArray<nsIContent> boundElements;
   nsBindingList bindings;
 
   for (auto iter = mBoundContentSet->Iter(); !iter.Done(); iter.Next()) {
     nsXBLBinding* binding = iter.Get()->GetKey()->GetXBLBinding();
     if (binding && bindings.AppendElement(binding)) {
       if (!boundElements.AppendObject(binding->GetBoundElement())) {
-        bindings.RemoveElementAt(bindings.Length() - 1);
+        bindings.RemoveLastElement();
       }
     }
   }
 
   uint32_t i, count = bindings.Length();
   for (i = 0; i < count; ++i) {
     bindings[i]->ExecuteDetachedHandler();
   }

--- a/dom/xslt/base/txNamespaceMap.cpp
+++ b/dom/xslt/base/txNamespaceMap.cpp
@@ -52,17 +52,17 @@ txNamespaceMap::mapNamespace(nsAtom* aPr
     }
 
     // New mapping
     if (!mPrefixes.AppendElement(prefix)) {
         return NS_ERROR_OUT_OF_MEMORY;
     }
 
     if (mNamespaces.AppendElement(nsId) == nullptr) {
-        mPrefixes.RemoveElementAt(mPrefixes.Length() - 1);
+        mPrefixes.RemoveLastElement();
 
         return NS_ERROR_OUT_OF_MEMORY;
     }
 
     return NS_OK;
 }
 
 int32_t

--- a/dom/xslt/xslt/txExecutionState.cpp
+++ b/dom/xslt/xslt/txExecutionState.cpp
@@ -391,17 +391,17 @@ txExecutionState::pushTemplateRule(txSty
     rule->mModeLocalName = aMode.mLocalName;
     rule->mParams = aParams;
 }
 
 void
 txExecutionState::popTemplateRule()
 {
     NS_PRECONDITION(!mTemplateRules.IsEmpty(), "No rules to pop");
-    mTemplateRules.RemoveElementAt(mTemplateRules.Length() - 1);
+    mTemplateRules.RemoveLastElement();
 }
 
 txIEvalContext*
 txExecutionState::getEvalContext()
 {
     return mEvalContext;
 }
 
@@ -529,13 +529,12 @@ txExecutionState::pushParamMap(txParamet
     mParamStack.AppendElement(mTemplateParams.forget());
     mTemplateParams = aParams;
 }
 
 already_AddRefed<txParameterMap>
 txExecutionState::popParamMap()
 {
     RefPtr<txParameterMap> oldParams = mTemplateParams.forget();
-    mTemplateParams = mParamStack.LastElement();
-    mParamStack.RemoveElementAt(mParamStack.Length() - 1);
+    mTemplateParams = mParamStack.PopLastElement();
 
     return oldParams.forget();
 }

--- a/extensions/spellcheck/src/mozInlineSpellWordUtil.cpp
+++ b/extensions/spellcheck/src/mozInlineSpellWordUtil.cpp
@@ -840,17 +840,17 @@ mozInlineSpellWordUtil::BuildSoftText()
         int32_t len = lastOffsetInNode - firstOffsetInNode;
         mSoftTextDOMMapping.AppendElement(
           DOMTextMapping(NodeOffset(node, firstOffsetInNode), mSoftText.Length(), len));
 
         bool ok = textFragment->AppendTo(mSoftText, firstOffsetInNode, len,
                                          mozilla::fallible);
         if (!ok) {
             // probably out of memory, remove from mSoftTextDOMMapping
-            mSoftTextDOMMapping.RemoveElementAt(mSoftTextDOMMapping.Length() - 1);
+            mSoftTextDOMMapping.RemoveLastElement();
             exit = true;
         }
       }
 
       firstOffsetInNode = 0;
     }
 
     if (exit)

--- a/gfx/gl/GLTextureImage.cpp
+++ b/gfx/gl/GLTextureImage.cpp
@@ -455,17 +455,17 @@ void TiledTextureImage::Resize(const gfx
                 mImages.RemoveElementAt(i);
             }
         }
     }
 
     // Prune any unused tiles at the end of the store.
     unsigned int length = mImages.Length();
     for (; i < length; i++)
-      mImages.RemoveElementAt(mImages.Length()-1);
+      mImages.RemoveLastElement();
 
     // Reset tile-store properties.
     mRows = rows;
     mColumns = columns;
     mSize = aSize;
     mTextureState = Allocated;
     mCurrentImage = 0;
 }

--- a/gfx/thebes/gfxContext.cpp
+++ b/gfx/thebes/gfxContext.cpp
@@ -165,17 +165,17 @@ gfxContext::Restore()
                "Save() been called. you may consider to remove this pair of "
                "gfxContext::Save/Restore.");
 #endif
 
   for (unsigned int c = 0; c < CurrentState().pushedClips.Length(); c++) {
     mDT->PopClip();
   }
 
-  mStateStack.RemoveElementAt(mStateStack.Length() - 1);
+  mStateStack.RemoveLastElement();
 
   mDT = CurrentState().drawTarget;
 
   ChangeTransform(CurrentState().transform, false);
 }
 
 // drawing
 void
@@ -604,17 +604,17 @@ gfxContext::Clip()
   }
 }
 
 void
 gfxContext::PopClip()
 {
   MOZ_ASSERT(CurrentState().pushedClips.Length() > 0);
 
-  CurrentState().pushedClips.RemoveElementAt(CurrentState().pushedClips.Length() - 1);
+  CurrentState().pushedClips.RemoveLastElement();
   mDT->PopClip();
 }
 
 gfxRect
 gfxContext::GetClipExtents(ClipExtentsSpace aSpace) const
 {
   Rect rect = GetAzureDeviceSpaceClipBounds();
 

--- a/gfx/vr/gfxVRPuppet.cpp
+++ b/gfx/vr/gfxVRPuppet.cpp
@@ -743,17 +743,17 @@ VRSystemManagerPuppet::Enumerate()
   while (mPuppetHMDs.Length() < mPuppetDisplayCount) {
     VRDisplayPuppet* puppetDisplay = new VRDisplayPuppet();
     uint32_t deviceID = mPuppetHMDs.Length();
     puppetDisplay->SetDisplayInfo(mPuppetDisplayInfo[deviceID]);
     puppetDisplay->SetSensorState(mPuppetDisplaySensorState[deviceID]);
     mPuppetHMDs.AppendElement(puppetDisplay);
   }
   while (mPuppetHMDs.Length() > mPuppetDisplayCount) {
-    mPuppetHMDs.RemoveElementAt(mPuppetHMDs.Length() - 1);
+    mPuppetHMDs.RemoveLastElement();
   }
 }
 
 void
 VRSystemManagerPuppet::SetPuppetDisplayInfo(const uint32_t& aDeviceID,
                                             const VRDisplayInfo& aDisplayInfo)
 {
   if (aDeviceID >= mPuppetDisplayCount) {

--- a/image/DecodePool.cpp
+++ b/image/DecodePool.cpp
@@ -222,18 +222,17 @@ private:
   ~DecodePoolImpl() { }
 
   bool CreateThread();
 
   Work PopWorkFromQueue(nsTArray<RefPtr<IDecodingTask>>& aQueue)
   {
     Work work;
     work.mType = Work::Type::TASK;
-    work.mTask = aQueue.LastElement().forget();
-    aQueue.RemoveElementAt(aQueue.Length() - 1);
+    work.mTask = aQueue.PopLastElement();
 
     return work;
   }
 
   Work CreateShutdownWork() const
   {
     Work work;
     work.mType = Work::Type::SHUTDOWN;

--- a/image/imgLoader.cpp
+++ b/image/imgLoader.cpp
@@ -1020,17 +1020,17 @@ imgCacheQueue::Remove(imgCacheEntry* ent
   mSize -= mQueue[index]->GetDataSize();
 
   // If the queue is clean and this is the first entry,
   // then we can efficiently remove the entry without
   // dirtying the sort order.
   if (!IsDirty() && index == 0) {
     std::pop_heap(mQueue.begin(), mQueue.end(),
                   imgLoader::CompareCacheEntries);
-    mQueue.RemoveElementAt(mQueue.Length() - 1);
+    mQueue.RemoveLastElement();
     return;
   }
 
   // Remove from the middle of the list.  This potentially
   // breaks the binary heap sort order.
   mQueue.RemoveElementAt(index);
 
   // If we only have one entry or the queue is empty, though,
@@ -1066,18 +1066,17 @@ imgCacheQueue::Pop()
   if (mQueue.IsEmpty()) {
     return nullptr;
   }
   if (IsDirty()) {
     Refresh();
   }
 
   std::pop_heap(mQueue.begin(), mQueue.end(), imgLoader::CompareCacheEntries);
-  RefPtr<imgCacheEntry> entry = Move(mQueue.LastElement());
-  mQueue.RemoveElementAt(mQueue.Length() - 1);
+  RefPtr<imgCacheEntry> entry = mQueue.PopLastElement();
 
   mSize -= entry->GetDataSize();
   return entry.forget();
 }
 
 void
 imgCacheQueue::Refresh()
 {

--- a/ipc/mscom/Utils.cpp
+++ b/ipc/mscom/Utils.cpp
@@ -335,18 +335,17 @@ IsInterfaceEqualToOrInheritedFrom(REFIID
     typeInfos.AppendElement(Move(typeInfo));
   }
 
   /**
    * The main loop of this function searches the hierarchy of aInterface's
    * parent interfaces, searching for aFrom.
    */
   while (!typeInfos.IsEmpty()) {
-    RefPtr<ITypeInfo> curTypeInfo(Move(typeInfos.LastElement()));
-    typeInfos.RemoveElementAt(typeInfos.Length() - 1);
+    RefPtr<ITypeInfo> curTypeInfo(typeInfos.PopLastElement());
 
     TYPEATTR* typeAttr = nullptr;
     HRESULT hr = curTypeInfo->GetTypeAttr(&typeAttr);
     if (FAILED(hr)) {
       break;
     }
 
     bool isFromParentVtable = IsVtableIndexFromParentInterface(typeAttr,

--- a/layout/base/PresShell.cpp
+++ b/layout/base/PresShell.cpp
@@ -2690,18 +2690,17 @@ PresShell::FrameNeedsReflow(nsIFrame *aF
     }
   }
 #endif
 
   AutoTArray<nsIFrame*, 4> subtrees;
   subtrees.AppendElement(aFrame);
 
   do {
-    nsIFrame *subtreeRoot = subtrees.ElementAt(subtrees.Length() - 1);
-    subtrees.RemoveElementAt(subtrees.Length() - 1);
+    nsIFrame *subtreeRoot = subtrees.PopLastElement();
 
     // Grab |wasDirty| now so we can go ahead and update the bits on
     // subtreeRoot.
     bool wasDirty = NS_SUBTREE_DIRTY(subtreeRoot);
     subtreeRoot->AddStateBits(aBitToAdd);
 
     // Determine whether we need to keep looking for the next ancestor
     // reflow root if subtreeRoot itself is a reflow root.
@@ -2741,18 +2740,17 @@ PresShell::FrameNeedsReflow(nsIFrame *aF
       // Mark all descendants dirty (using an nsTArray stack rather than
       // recursion).
       // Note that ReflowInput::InitResizeFlags has some similar
       // code; see comments there for how and why it differs.
       AutoTArray<nsIFrame*, 32> stack;
       stack.AppendElement(subtreeRoot);
 
       do {
-        nsIFrame *f = stack.ElementAt(stack.Length() - 1);
-        stack.RemoveElementAt(stack.Length() - 1);
+        nsIFrame *f = stack.PopLastElement();
 
         if (f->IsPlaceholderFrame()) {
           nsIFrame *oof = nsPlaceholderFrame::GetRealFrameForPlaceholder(f);
           if (!nsLayoutUtils::IsProperAncestorFrame(subtreeRoot, oof)) {
             // We have another distinct subtree we need to mark.
             subtrees.AppendElement(oof);
           }
         }

--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -3270,17 +3270,17 @@ static bool gDumpEventList = false;
 // maintaining a single paint count, we need a stack.
 StaticAutoPtr<nsTArray<int>> gPaintCountStack;
 
 struct AutoNestedPaintCount {
   AutoNestedPaintCount() {
     gPaintCountStack->AppendElement(0);
   }
   ~AutoNestedPaintCount() {
-    gPaintCountStack->RemoveElementAt(gPaintCountStack->Length() - 1);
+    gPaintCountStack->RemoveLastElement();
   }
 };
 
 #endif
 
 nsIFrame*
 nsLayoutUtils::GetFrameForPoint(nsIFrame* aFrame, nsPoint aPt, uint32_t aFlags)
 {
@@ -5823,29 +5823,27 @@ nsLayoutUtils::ComputeBSizeDependentValu
 nsLayoutUtils::MarkDescendantsDirty(nsIFrame *aSubtreeRoot)
 {
   AutoTArray<nsIFrame*, 4> subtrees;
   subtrees.AppendElement(aSubtreeRoot);
 
   // dirty descendants, iterating over subtrees that may include
   // additional subtrees associated with placeholders
   do {
-    nsIFrame *subtreeRoot = subtrees.ElementAt(subtrees.Length() - 1);
-    subtrees.RemoveElementAt(subtrees.Length() - 1);
+    nsIFrame *subtreeRoot = subtrees.PopLastElement();
 
     // Mark all descendants dirty (using an nsTArray stack rather than
     // recursion).
     // Note that ReflowInput::InitResizeFlags has some similar
     // code; see comments there for how and why it differs.
     AutoTArray<nsIFrame*, 32> stack;
     stack.AppendElement(subtreeRoot);
 
     do {
-      nsIFrame *f = stack.ElementAt(stack.Length() - 1);
-      stack.RemoveElementAt(stack.Length() - 1);
+      nsIFrame *f = stack.PopLastElement();
 
       f->MarkIntrinsicISizesDirty();
 
       if (f->IsPlaceholderFrame()) {
         nsIFrame *oof = nsPlaceholderFrame::GetRealFrameForPlaceholder(f);
         if (!nsLayoutUtils::IsProperAncestorFrame(subtreeRoot, oof)) {
           // We have another distinct subtree we need to mark.
           subtrees.AppendElement(oof);
@@ -5867,18 +5865,17 @@ nsLayoutUtils::MarkDescendantsDirty(nsIF
 /* static */
 void
 nsLayoutUtils::MarkIntrinsicISizesDirtyIfDependentOnBSize(nsIFrame* aFrame)
 {
   AutoTArray<nsIFrame*, 32> stack;
   stack.AppendElement(aFrame);
 
   do {
-    nsIFrame* f = stack.ElementAt(stack.Length() - 1);
-    stack.RemoveElementAt(stack.Length() - 1);
+    nsIFrame* f = stack.PopLastElement();
 
     if (!f->HasAnyStateBits(
         NS_FRAME_DESCENDANT_INTRINSIC_ISIZE_DEPENDS_ON_BSIZE)) {
       continue;
     }
     f->MarkIntrinsicISizesDirty();
 
     for (nsIFrame::ChildListIterator lists(f); !lists.IsDone(); lists.Next()) {

--- a/layout/base/nsStyleChangeList.h
+++ b/layout/base/nsStyleChangeList.h
@@ -46,17 +46,17 @@ public:
   ~nsStyleChangeList() { MOZ_COUNT_DTOR(nsStyleChangeList); }
   void AppendChange(nsIFrame* aFrame, nsIContent* aContent, nsChangeHint aHint);
 
   // Starting from the end of the list, removes all changes until the list is
   // empty or an element with |mContent != aContent| is found.
   void PopChangesForContent(nsIContent* aContent)
   {
     while (!IsEmpty() && LastElement().mContent == aContent) {
-      RemoveElementAt(Length() - 1);
+      RemoveLastElement();
     }
   }
 
   bool IsGecko() const { return mType == mozilla::StyleBackendType::Gecko; }
   bool IsServo() const { return mType == mozilla::StyleBackendType::Servo; }
 
 private:
   mozilla::StyleBackendType mType;

--- a/layout/generic/ReflowInput.cpp
+++ b/layout/generic/ReflowInput.cpp
@@ -647,18 +647,17 @@ ReflowInput::InitResizeFlags(nsPresConte
       // need to because we're actually doing something that cares about
       // frame tree geometry (the width on an ancestor) rather than
       // style.
 
       AutoTArray<nsIFrame*, 32> stack;
       stack.AppendElement(mFrame);
 
       do {
-        nsIFrame *f = stack.ElementAt(stack.Length() - 1);
-        stack.RemoveElementAt(stack.Length() - 1);
+        nsIFrame *f = stack.PopLastElement();
 
         nsIFrame::ChildListIterator lists(f);
         for (; !lists.IsDone(); lists.Next()) {
           nsFrameList::Enumerator childFrames(lists.CurrentList());
           for (; !childFrames.AtEnd(); childFrames.Next()) {
             nsIFrame* kid = childFrames.get();
             kid->MarkIntrinsicISizesDirty();
             stack.AppendElement(kid);

--- a/layout/generic/TextDrawTarget.h
+++ b/layout/generic/TextDrawTarget.h
@@ -180,17 +180,17 @@ public:
   PushClipRect(const Rect &aRect) override {
     LayoutDeviceRect rect = LayoutDeviceRect::FromUnknownRect(aRect);
     rect = rect.Intersect(mClipStack.LastElement());
     mClipStack.AppendElement(rect);
   }
 
   void
   PopClip() override {
-    mClipStack.RemoveElementAt(mClipStack.Length() - 1);
+    mClipStack.RemoveLastElement();
   }
 
   void
   AppendShadow(const wr::Shadow& aShadow)
   {
     mBuilder.PushShadow(mBoundsRect, ClipRect(), mBackfaceVisible, aShadow);
     mHasShadows = true;
   }

--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -12204,17 +12204,17 @@ DR_FrameTreeNode* DR_State::CreateTreeNo
   FindMatchingRule(*newNode);
 
   newNode->mIndent = mIndent;
   if (newNode->mDisplay || mIndentUndisplayedFrames) {
     ++mIndent;
   }
 
   if (lastLeaf && (lastLeaf == parentNode)) {
-    mFrameTreeLeaves.RemoveElementAt(mFrameTreeLeaves.Length() - 1);
+    mFrameTreeLeaves.RemoveLastElement();
   }
   mFrameTreeLeaves.AppendElement(newNode);
   mCount++;
 
   return newNode;
 }
 
 void DR_State::PrettyUC(nscoord aSize,

--- a/layout/painting/RetainedDisplayListBuilder.cpp
+++ b/layout/painting/RetainedDisplayListBuilder.cpp
@@ -194,17 +194,17 @@ template<typename T>
 void SwapAndRemove(nsTArray<T>& aArray, uint32_t aIndex)
 {
   if (aIndex != (aArray.Length() - 1)) {
     T last = aArray.LastElement();
     aArray.LastElement() = aArray[aIndex];
     aArray[aIndex] = last;
   }
 
-  aArray.RemoveElementAt(aArray.Length() - 1);
+  aArray.RemoveLastElement();
 }
 
 static bool
 MergeFrameRects(nsDisplayLayerEventRegions* aOldItem,
                 nsDisplayLayerEventRegions* aNewItem,
                 nsDisplayLayerEventRegions::FrameRects nsDisplayLayerEventRegions::*aRectList,
                 nsTArray<nsIFrame*>& aAddedFrames)
 {

--- a/layout/painting/nsDisplayList.h
+++ b/layout/painting/nsDisplayList.h
@@ -6778,18 +6778,17 @@ private:
   void ResolveFlattening()
   {
     // Handle the case where we reach the end of a nested list, or the current
     // item should start a new nested list. Repeat this until we find an actual
     // item, or the very end of the outer list.
     while (AtEndOfNestedList() || ShouldFlattenNextItem()) {
       if (AtEndOfNestedList()) {
         // Pop the last item off the stack.
-        mNext = mStack.LastElement();
-        mStack.RemoveElementAt(mStack.Length() - 1);
+        mNext = mStack.PopLastElement();
         // We stored the item that was flattened, so advance to the next.
         mNext = mNext->GetAbove();
       } else {
         // This item wants to be flattened. Store the current item on the stack,
         // and use the first item in the child list instead.
         mStack.AppendElement(mNext);
         nsDisplayList* childItems = mNext->GetSameCoordinateSystemChildren();
         mNext = childItems->GetBottom();

--- a/layout/style/FontFaceSet.cpp
+++ b/layout/style/FontFaceSet.cpp
@@ -1224,17 +1224,17 @@ FontFaceSet::FindOrCreateUserFontEntryFr
               // unknown format specified, mark to distinguish from the
               // case where no format hints are specified
               face->mFormatFlags |= gfxUserFontSet::FLAG_FORMAT_UNKNOWN;
             }
             i++;
           }
           if (!face->mURI) {
             // if URI not valid, omit from src array
-            srcArray.RemoveElementAt(srcArray.Length() - 1);
+            srcArray.RemoveLastElement();
             NS_WARNING("null url in @font-face rule");
             continue;
           }
           break;
         }
         default:
           NS_ASSERTION(unit == eCSSUnit_Local_Font || unit == eCSSUnit_URL,
                        "strange unit type in font-face src array");

--- a/layout/style/nsFontFaceUtils.cpp
+++ b/layout/style/nsFontFaceUtils.cpp
@@ -110,26 +110,24 @@ nsFontFaceUtils::MarkDirtyForFontChange(
   AutoTArray<nsIFrame*, 4> subtrees;
   subtrees.AppendElement(aSubtreeRoot);
 
   nsIPresShell* ps = aSubtreeRoot->PresShell();
 
   // check descendants, iterating over subtrees that may include
   // additional subtrees associated with placeholders
   do {
-    nsIFrame* subtreeRoot = subtrees.ElementAt(subtrees.Length() - 1);
-    subtrees.RemoveElementAt(subtrees.Length() - 1);
+    nsIFrame* subtreeRoot = subtrees.PopLastElement();
 
     // Check all descendants to see if they use the font
     AutoTArray<nsIFrame*, 32> stack;
     stack.AppendElement(subtreeRoot);
 
     do {
-      nsIFrame* f = stack.ElementAt(stack.Length() - 1);
-      stack.RemoveElementAt(stack.Length() - 1);
+      nsIFrame* f = stack.PopLastElement();
 
       // if this frame uses the font, mark its descendants dirty
       // and skip checking its children
       if (FrameUsesFont(f, aFont)) {
         ScheduleReflow(ps, f);
       } else {
         if (f->IsPlaceholderFrame()) {
           nsIFrame* oof = nsPlaceholderFrame::GetRealFrameForPlaceholder(f);

--- a/netwerk/cache2/CacheIndexIterator.cpp
+++ b/netwerk/cache2/CacheIndexIterator.cpp
@@ -39,17 +39,17 @@ CacheIndexIterator::GetNextHash(SHA1Sum:
   }
 
   if (!mRecords.Length()) {
     CloseInternal(NS_ERROR_NOT_AVAILABLE);
     return mStatus;
   }
 
   memcpy(aHash, mRecords[mRecords.Length() - 1]->mHash, sizeof(SHA1Sum::Hash));
-  mRecords.RemoveElementAt(mRecords.Length() - 1);
+  mRecords.RemoveLastElement();
 
   return NS_OK;
 }
 
 nsresult
 CacheIndexIterator::Close()
 {
   LOG(("CacheIndexIterator::Close() [this=%p]", this));

--- a/netwerk/protocol/http/nsHttpChannel.cpp
+++ b/netwerk/protocol/http/nsHttpChannel.cpp
@@ -8251,18 +8251,17 @@ nsHttpChannel::OnRedirectVerifyCallback(
     mWaitingForRedirectCallback = false;
 
     if (mCanceled && NS_SUCCEEDED(result))
         result = NS_BINDING_ABORTED;
 
     for (uint32_t i = mRedirectFuncStack.Length(); i > 0;) {
         --i;
         // Pop the last function pushed to the stack
-        nsContinueRedirectionFunc func = mRedirectFuncStack[i];
-        mRedirectFuncStack.RemoveElementAt(mRedirectFuncStack.Length() - 1);
+        nsContinueRedirectionFunc func = mRedirectFuncStack.PopLastElement();
 
         // Call it with the result we got from the callback or the deeper
         // function call.
         result = (this->*func)(result);
 
         // If a new function has been pushed to the stack and placed us in the
         // waiting state, we need to break the chain and wait for the callback
         // again.

--- a/parser/html/nsHtml5Highlighter.cpp
+++ b/parser/html/nsHtml5Highlighter.cpp
@@ -705,17 +705,17 @@ nsHtml5Highlighter::Push(nsAtom* aName,
   mOpQueue.AppendElement()->Init(eTreeOpAppend, elt, CurrentNode());
   mStack.AppendElement(elt);
 }
 
 void
 nsHtml5Highlighter::Pop()
 {
   NS_PRECONDITION(mStack.Length() >= 2, "Popping when stack too short.");
-  mStack.RemoveElementAt(mStack.Length() - 1);
+  mStack.RemoveLastElement();
 }
 
 void
 nsHtml5Highlighter::AppendCharacters(const char16_t* aBuffer,
                                      int32_t aStart,
                                      int32_t aLength)
 {
   NS_PRECONDITION(aBuffer, "Null buffer");

--- a/toolkit/components/places/History.cpp
+++ b/toolkit/components/places/History.cpp
@@ -3243,17 +3243,17 @@ History::UpdatePlaces(JS::Handle<JS::Val
                           nsINavHistoryService::TRANSITION_RELOAD);
       data.SetTransitionType(transitionType);
       data.hidden = GetHiddenState(false, transitionType);
 
       // If the visit is an embed visit, we do not actually add it to the
       // database.
       if (transitionType == nsINavHistoryService::TRANSITION_EMBED) {
         StoreAndNotifyEmbedVisit(data, aCallback);
-        visitData.RemoveElementAt(visitData.Length() - 1);
+        visitData.RemoveLastElement();
         initialUpdatedCount++;
         continue;
       }
 
       // The referrer is optional.
       nsCOMPtr<nsIURI> referrer = GetURIFromJSObject(aCtx, visit,
                                                      "referrerURI");
       if (referrer) {

--- a/widget/cocoa/TextInputHandler.h
+++ b/widget/cocoa/TextInputHandler.h
@@ -874,18 +874,17 @@ protected:
   /**
    * RemoveCurrentKeyEvent() removes the current key event from
    * mCurrentKeyEvents.
    */
   void RemoveCurrentKeyEvent()
   {
     NS_ASSERTION(mCurrentKeyEvents.Length() > 0,
                  "RemoveCurrentKeyEvent() is called unexpectedly");
-    KeyEventState* keyEvent = GetCurrentKeyEvent();
-    mCurrentKeyEvents.RemoveElementAt(mCurrentKeyEvents.Length() - 1);
+    KeyEventState* keyEvent = mCurrentKeyEvents.PopLastElement();
     if (keyEvent == &mFirstKeyEvent) {
       keyEvent->Clear();
     } else {
       delete keyEvent;
     }
   }
 
   /**

--- a/widget/windows/TSFTextStore.cpp
+++ b/widget/windows/TSFTextStore.cpp
@@ -5126,17 +5126,17 @@ TSFTextStore::RecordCompositionStartActi
   // compositionend and start composition normally.
   if (!aPreserveSelection &&
       WasTextInsertedWithoutCompositionAt(aStart, aLength)) {
     const PendingAction& pendingCompositionEnd = mPendingActions.LastElement();
     const PendingAction& pendingCompositionStart =
       mPendingActions[mPendingActions.Length() - 2];
     contentForTSF.RestoreCommittedComposition(
       aComposition, pendingCompositionStart, pendingCompositionEnd);
-    mPendingActions.RemoveElementAt(mPendingActions.Length() - 1);
+    mPendingActions.RemoveLastElement();
     MOZ_LOG(sTextStoreLog, LogLevel::Info,
       ("0x%p   TSFTextStore::RecordCompositionStartAction() "
        "succeeded: restoring the committed string as composing string, "
        "mComposition={ mStart=%ld, mString.Length()=%ld, "
        "mSelectionForTSF={ acpStart=%ld, acpEnd=%ld, style.ase=%s, "
        "style.fInterimChar=%s } }",
        this, mComposition.mStart, mComposition.mString.Length(),
        mSelectionForTSF.StartOffset(), mSelectionForTSF.EndOffset(),

--- a/xpcom/threads/DeadlockDetector.h
+++ b/xpcom/threads/DeadlockDetector.h
@@ -342,17 +342,17 @@ public:
 
     index_type i = 0;
     size_type len = aStart->mOrderedLT.Length();
     for (auto it = aStart->mOrderedLT.Elements(); i < len; ++i, ++it) {
       aChain->AppendElement((*it)->mResource);
       if (GetDeductionChain_Helper(*it, aTarget, aChain)) {
         return true;
       }
-      aChain->RemoveElementAt(aChain->Length() - 1);
+      aChain->RemoveLastElement();
     }
     return false;
   }
 
   /**
    * The partial order on resource acquisitions used by the deadlock
    * detector.
    */

--- a/xpcom/threads/ThreadEventQueue.cpp
+++ b/xpcom/threads/ThreadEventQueue.cpp
@@ -252,17 +252,17 @@ ThreadEventQueue<InnerQueueT>::PopEventQ
 
   // Move events from the old queue to the new one.
   nsCOMPtr<nsIRunnable> event;
   EventPriority prio;
   while ((event = item.mQueue->GetEvent(&prio, lock))) {
     prevQueue->PutEvent(event.forget(), prio, lock);
   }
 
-  mNestedQueues.RemoveElementAt(mNestedQueues.Length() - 1);
+  mNestedQueues.RemoveLastElement();
 }
 
 template<class InnerQueueT>
 already_AddRefed<nsIThreadObserver>
 ThreadEventQueue<InnerQueueT>::GetObserver()
 {
   MutexAutoLock lock(mLock);
   return do_AddRef(mObserver);

--- a/xpcom/threads/TimerThread.cpp
+++ b/xpcom/threads/TimerThread.cpp
@@ -734,17 +734,17 @@ TimerThread::RemoveLeadingCanceledTimers
 }
 
 void
 TimerThread::RemoveFirstTimerInternal()
 {
   mMonitor.AssertCurrentThreadOwns();
   MOZ_ASSERT(!mTimers.IsEmpty());
   std::pop_heap(mTimers.begin(), mTimers.end(), Entry::UniquePtrLessThan);
-  mTimers.RemoveElementAt(mTimers.Length() - 1);
+  mTimers.RemoveLastElement();
 }
 
 already_AddRefed<nsTimerImpl>
 TimerThread::PostTimerEvent(already_AddRefed<nsTimerImpl> aTimerRef)
 {
   mMonitor.AssertCurrentThreadOwns();
 
   RefPtr<nsTimerImpl> timer(aTimerRef);