--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -3387,18 +3387,16 @@ nsCSSFrameConstructor::ConstructFieldSet
   }
 
   // Set the inner frame's initial child lists
   contentFrame->SetInitialChildList(kPrincipalList, childItems);
 
   // Set the outer frame's initial child list
   fieldsetFrame->SetInitialChildList(kPrincipalList, fieldsetKids);
 
-  fieldsetFrame->AddStateBits(NS_FRAME_MAY_HAVE_GENERATED_CONTENT);
-
   // Our new frame returned is the outer frame, which is the fieldset frame.
   return fieldsetFrame;
 }
 
 nsIFrame*
 nsCSSFrameConstructor::ConstructDetailsFrame(nsFrameConstructorState& aState,
                                              FrameConstructionItem& aItem,
                                              nsContainerFrame* aParentFrame,
@@ -5739,20 +5737,16 @@ nsCSSFrameConstructor::AddFrameConstruct
 
   // Pre-check for display "none" - if we find that, don't create
   // any frame at all
   if (display->mDisplay == StyleDisplay::None) {
     return;
   }
 
   if (display->mDisplay == StyleDisplay::Contents) {
-    if (aParentFrame) {
-      // FIXME(emilio): Pretty sure aParentFrame can't be null here.
-      aParentFrame->AddStateBits(NS_FRAME_MAY_HAVE_GENERATED_CONTENT);
-    }
     CreateGeneratedContentItem(aState, aParentFrame, aContent->AsElement(),
                                computedStyle, CSSPseudoElementType::before,
                                aItems);
 
     FlattenedChildIterator iter(aContent);
     for (nsIContent* child = iter.GetNextChild(); child; child = iter.GetNextChild()) {
       if (!ShouldCreateItemsForChild(aState, child, aParentFrame)) {
         continue;
@@ -10302,17 +10296,16 @@ nsCSSFrameConstructor::ProcessChildren(n
     // :before/:after content should have the same style parent as normal kids.
     //
     // Note that we don't use this style for looking up things like special
     // block styles because in some cases involving table pseudo-frames it has
     // nothing to do with the parent frame's desired behavior.
     ComputedStyle* computedStyle;
 
     if (aCanHaveGeneratedContent) {
-      aFrame->AddStateBits(NS_FRAME_MAY_HAVE_GENERATED_CONTENT);
       computedStyle =
         nsFrame::CorrectStyleParentFrame(aFrame, nullptr)->Style();
       // Probe for generated content before
       CreateGeneratedContentItem(aState, aFrame, aContent->AsElement(),
                                  computedStyle, CSSPseudoElementType::before,
                                  itemsToConstruct);
     }
 
@@ -11368,19 +11361,16 @@ nsCSSFrameConstructor::ConstructInline(n
     aDisplay->IsRelativelyPositionedStyle() &&
     !nsSVGUtils::IsInSVGTextSubtree(aParentFrame);
 
   nsInlineFrame* newFrame = NS_NewInlineFrame(mPresShell, computedStyle);
 
   // Initialize the frame
   InitAndRestoreFrame(aState, content, aParentFrame, newFrame);
 
-  // Inline frames can always have generated content
-  newFrame->AddStateBits(NS_FRAME_MAY_HAVE_GENERATED_CONTENT);
-
   nsFrameConstructorSaveState absoluteSaveState;  // definition cannot be inside next block
                                                   // because the object's destructor is significant
                                                   // this is part of the fix for bug 42372
 
   newFrame->AddStateBits(NS_FRAME_CAN_HAVE_ABSPOS_CHILDREN);
   if (positioned) {
     // Relatively positioned frames becomes a container for child
     // frames that are positioned
@@ -11473,18 +11463,17 @@ nsCSSFrameConstructor::CreateIBSiblings(
 
     SetFrameIsIBSplit(lastNewInline, blockFrame);
     aSiblings.AddChild(blockFrame);
 
     // Now grab the initial inlines in aChildItems and put them into an inline
     // frame.
     nsInlineFrame* inlineFrame = NS_NewInlineFrame(mPresShell, computedStyle);
     InitAndRestoreFrame(aState, content, parentFrame, inlineFrame, false);
-    inlineFrame->AddStateBits(NS_FRAME_MAY_HAVE_GENERATED_CONTENT |
-                              NS_FRAME_CAN_HAVE_ABSPOS_CHILDREN);
+    inlineFrame->AddStateBits(NS_FRAME_CAN_HAVE_ABSPOS_CHILDREN);
     if (aIsPositioned) {
       inlineFrame->MarkAsAbsoluteContainingBlock();
     }
 
     if (aChildItems.NotEmpty()) {
       nsFrameList::FrameLinkEnumerator firstBlock(aChildItems);
       FindFirstBlock(firstBlock);
       nsFrameList inlineKids = aChildItems.ExtractHead(firstBlock);

--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -629,17 +629,16 @@ nsFrame::Init(nsIContent*       aContent
 
     // Make sure the general flags bits are the same
     nsFrameState state = aPrevInFlow->GetStateBits();
 
     // Make bits that are currently off (see constructor) the same:
     AddStateBits(state & (NS_FRAME_INDEPENDENT_SELECTION |
                           NS_FRAME_PART_OF_IBSPLIT |
                           NS_FRAME_MAY_BE_TRANSFORMED |
-                          NS_FRAME_MAY_HAVE_GENERATED_CONTENT |
                           NS_FRAME_CAN_HAVE_ABSPOS_CHILDREN));
   } else {
     PresContext()->ConstructedFrame();
   }
   if (GetParent()) {
     nsFrameState state = GetParent()->GetStateBits();
 
     // Make bits that are currently off (see constructor) the same:

--- a/layout/generic/nsFrameStateBits.h
+++ b/layout/generic/nsFrameStateBits.h
@@ -219,20 +219,18 @@ FRAME_STATE_BIT(Generic, 41, NS_FRAME_FO
 // context?
 FRAME_STATE_BIT(Generic, 42, NS_FRAME_FONT_INFLATION_FLOW_ROOT)
 
 // This bit is set on SVG frames that are laid out using SVG's coordinate
 // system based layout (as opposed to any of the CSS layout models). Note that
 // this does not include nsSVGOuterSVGFrame since it takes part is CSS layout.
 FRAME_STATE_BIT(Generic, 43, NS_FRAME_SVG_LAYOUT)
 
-// Is this frame allowed to have generated (::before/::after) content?
-FRAME_STATE_BIT(Generic, 44, NS_FRAME_MAY_HAVE_GENERATED_CONTENT)
-
-// Bit 45 is currently unused.
+// Bits 44 and 45 are currently unused, but be kind and check with bug 1465478
+// first please :-)
 
 // This bit indicates that we're tracking visibility for this frame, and that
 // the frame has a VisibilityStateProperty property.
 FRAME_STATE_BIT(Generic, 46, NS_FRAME_VISIBILITY_IS_TRACKED)
 
 // The frame is a descendant of SVGTextFrame and is thus used for SVG
 // text layout.
 FRAME_STATE_BIT(Generic, 47, NS_FRAME_IS_SVG_TEXT)
@@ -282,17 +280,17 @@ FRAME_STATE_BIT(Generic, 58, NS_FRAME_MA
 // This state bit is set on frames within token MathML elements if the
 // token represents an <mi> tag whose inner HTML consists of a single
 // non-whitespace character to allow special rendering behaviour.
 FRAME_STATE_BIT(Generic, 59, NS_FRAME_IS_IN_SINGLE_CHAR_MI)
 
 // NOTE: Bits 20-31 and 60-63 of the frame state are reserved for specific
 // frame classes.
 
-// NOTE: Bit 45 is currently unused and available.
+// NOTE: Bits 44 and 45 are currently unused.
 
 
 // == Frame state bits that apply to box frames ===============================
 
 FRAME_STATE_GROUP(Box, nsBoxFrame)
 
 FRAME_STATE_BIT(Box, 20, NS_STATE_BOX_CHILD_RESERVED)
 FRAME_STATE_BIT(Box, 21, NS_STATE_STACK_NOT_POSITIONED)