--- a/layout/generic/BlockReflowInput.cpp
+++ b/layout/generic/BlockReflowInput.cpp
@@ -983,19 +983,18 @@ BlockReflowInput::FlowAndPlaceFloat(nsIF
     nsFloatManager::CalculateRegionFor(wm, aFloat, floatMargin,
                                        ContainerSize());
   // if the float split, then take up all of the vertical height
   if (NS_FRAME_IS_NOT_COMPLETE(reflowStatus) &&
       (NS_UNCONSTRAINEDSIZE != ContentBSize())) {
     region.BSize(wm) = std::max(region.BSize(wm),
                                 ContentBSize() - floatPos.B(wm));
   }
-  DebugOnly<nsresult> rv = mFloatManager->AddFloat(aFloat, region, wm,
-                                                   ContainerSize());
-  MOZ_ASSERT(NS_SUCCEEDED(rv), "bad float placement");
+  mFloatManager->AddFloat(aFloat, region, wm, ContainerSize());
+
   // store region
   nsFloatManager::StoreRegionFor(wm, aFloat, region, ContainerSize());
 
   // If the float's dimensions have changed, note the damage in the
   // float manager.
   if (!region.IsEqualEdges(oldRegion)) {
     // XXXwaterson conservative: we could probably get away with noting
     // less damage; e.g., if only height has changed, then only note the

--- a/layout/generic/nsFloatManager.cpp
+++ b/layout/generic/nsFloatManager.cpp
@@ -256,17 +256,17 @@ nsFloatManager::GetFlowArea(WritingMode 
                         ? lineLeft - mLineLeft
                         : mLineLeft - lineRight +
                           LogicalSize(aWM, aContainerSize).ISize(aWM);
 
   return nsFlowAreaRect(aWM, inlineStart, blockStart - mBlockStart,
                         lineRight - lineLeft, blockSize, haveFloats);
 }
 
-nsresult
+void
 nsFloatManager::AddFloat(nsIFrame* aFloatFrame, const LogicalRect& aMarginRect,
                          WritingMode aWM, const nsSize& aContainerSize)
 {
   CHECK_BLOCK_AND_LINE_DIR(aWM);
   NS_ASSERTION(aMarginRect.ISize(aWM) >= 0, "negative inline size!");
   NS_ASSERTION(aMarginRect.BSize(aWM) >= 0, "negative block size!");
 
   FloatInfo info(aFloatFrame, mLineLeft, mBlockStart, aMarginRect, aWM,
@@ -285,20 +285,17 @@ nsFloatManager::AddFloat(nsIFrame* aFloa
   MOZ_ASSERT(floatStyle == StyleFloat::Left || floatStyle == StyleFloat::Right,
              "Unexpected float style!");
   nscoord& sideBEnd =
     floatStyle == StyleFloat::Left ? info.mLeftBEnd : info.mRightBEnd;
   nscoord thisBEnd = info.BEnd();
   if (thisBEnd > sideBEnd)
     sideBEnd = thisBEnd;
 
-  if (!mFloats.AppendElement(info))
-    return NS_ERROR_OUT_OF_MEMORY;
-
-  return NS_OK;
+  mFloats.AppendElement(Move(info));
 }
 
 // static
 LogicalRect
 nsFloatManager::CalculateRegionFor(WritingMode          aWM,
                                    nsIFrame*            aFloat,
                                    const LogicalMargin& aMargin,
                                    const nsSize&        aContainerSize)
@@ -572,22 +569,22 @@ nsFloatManager::FloatInfo::FloatInfo(nsI
 
     mShapeBoxRect.emplace(rect.LineLeft(aWM, aContainerSize) + aLineLeft,
                           rect.BStart(aWM) + aBlockStart,
                           rect.ISize(aWM), rect.BSize(aWM));
   }
 }
 
 #ifdef NS_BUILD_REFCNT_LOGGING
-nsFloatManager::FloatInfo::FloatInfo(const FloatInfo& aOther)
-  : mFrame(aOther.mFrame)
-  , mLeftBEnd(aOther.mLeftBEnd)
-  , mRightBEnd(aOther.mRightBEnd)
-  , mRect(aOther.mRect)
-  , mShapeBoxRect(aOther.mShapeBoxRect)
+nsFloatManager::FloatInfo::FloatInfo(FloatInfo&& aOther)
+  : mFrame(Move(aOther.mFrame))
+  , mLeftBEnd(Move(aOther.mLeftBEnd))
+  , mRightBEnd(Move(aOther.mRightBEnd))
+  , mRect(Move(aOther.mRect))
+  , mShapeBoxRect(Move(aOther.mShapeBoxRect))
 {
   MOZ_COUNT_CTOR(nsFloatManager::FloatInfo);
 }
 
 nsFloatManager::FloatInfo::~FloatInfo()
 {
   MOZ_COUNT_DTOR(nsFloatManager::FloatInfo);
 }

--- a/layout/generic/nsFloatManager.h
+++ b/layout/generic/nsFloatManager.h
@@ -196,19 +196,19 @@ public:
   /**
    * Add a float that comes after all floats previously added.  Its
    * block start must be even with or below the top of all previous
    * floats.
    *
    * aMarginRect is relative to the current translation.  The caller
    * must ensure aMarginRect.height >= 0 and aMarginRect.width >= 0.
    */
-  nsresult AddFloat(nsIFrame* aFloatFrame,
-                    const mozilla::LogicalRect& aMarginRect,
-                    mozilla::WritingMode aWM, const nsSize& aContainerSize);
+  void AddFloat(nsIFrame* aFloatFrame,
+                const mozilla::LogicalRect& aMarginRect,
+                mozilla::WritingMode aWM, const nsSize& aContainerSize);
 
   /**
    * Notify that we tried to place a float that could not fit at all and
    * had to be pushed to the next page/column?  (If so, we can't place
    * any more floats in this page/column because of the rule that the
    * top of a float cannot be above the top of an earlier float.  It
    * also means that any clear needs to continue to the next column.)
    */
@@ -382,17 +382,17 @@ private:
       const nscoord aBStartCornerRadiusL, const nscoord aBStartCornerRadiusB,
       const nscoord aBEndCornerRadiusL, const nscoord aBEndCornerRadiusB,
       const nscoord aBandBStart, const nscoord aBandBEnd);
 
     static nscoord XInterceptAtY(const nscoord aY, const nscoord aRadiusX,
                                  const nscoord aRadiusY);
 
 #ifdef NS_BUILD_REFCNT_LOGGING
-    FloatInfo(const FloatInfo& aOther);
+    FloatInfo(FloatInfo&& aOther);
     ~FloatInfo();
 #endif
 
     // NB! This is really a logical rect in a writing mode suitable for
     // placing floats, which is not necessarily the actual writing mode
     // either of the block which created the frame manager or the block
     // that is calling the frame manager. The inline coordinates are in
     // the line-relative axis of the frame manager and its block