--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -485,30 +485,40 @@ AnyKidsNeedBlockParent(nsIFrame *aFrameL
   }
   return nullptr;
 }
 
 // Reparent a frame into a wrapper frame that is a child of its old parent.
 static void
 ReparentFrame(RestyleManager* aRestyleManager,
               nsContainerFrame* aNewParentFrame,
-              nsIFrame* aFrame)
+              nsIFrame* aFrame,
+              bool aForceStyleReparent)
 {
   aFrame->SetParent(aNewParentFrame);
-  aRestyleManager->ReparentStyleContext(aFrame);
+  // We reparent frames for two reasons: to put them inside ::first-line, and to
+  // put them inside some wrapper anonymous boxes.
+  //
+  // The latter shouldn't affect any styles in practice, so only needs style
+  // context reparenting in the Gecko backend, to make our style context tree
+  // assertions happy.  The former passes aForceStyleReparent == true.
+  if (aForceStyleReparent || aRestyleManager->IsGecko()) {
+    aRestyleManager->ReparentStyleContext(aFrame);
+  }
 }
 
 static void
 ReparentFrames(nsCSSFrameConstructor* aFrameConstructor,
                nsContainerFrame* aNewParentFrame,
-               const nsFrameList& aFrameList)
+               const nsFrameList& aFrameList,
+               bool aForceStyleReparent)
 {
   RestyleManager* restyleManager = aFrameConstructor->RestyleManager();
-  for (nsFrameList::Enumerator e(aFrameList); !e.AtEnd(); e.Next()) {
-    ReparentFrame(restyleManager, aNewParentFrame, e.get());
+  for (nsIFrame* f : aFrameList) {
+    ReparentFrame(restyleManager, aNewParentFrame, f, aForceStyleReparent);
   }
 }
 
 //----------------------------------------------------------------------
 //
 // When inline frames get weird and have block frames in them, we
 // annotate them to help us respond to incremental content changes
 // more easily.
@@ -5252,17 +5262,17 @@ nsCSSFrameConstructor::FlushAccumulatedB
   // then, create a block frame that will wrap the child frames. Make it a
   // MathML frame so that Get(Absolute/Float)ContainingBlockFor know that this
   // is not a suitable block.
   nsContainerFrame* blockFrame =
     NS_NewMathMLmathBlockFrame(mPresShell, blockContext);
   blockFrame->AddStateBits(NS_BLOCK_FORMATTING_CONTEXT_STATE_BITS);
 
   InitAndRestoreFrame(aState, aContent, aParentFrame, blockFrame);
-  ReparentFrames(this, blockFrame, aBlockItems);
+  ReparentFrames(this, blockFrame, aBlockItems, false);
   // abs-pos and floats are disabled in MathML children so we don't have to
   // worry about messing up those.
   blockFrame->SetInitialChildList(kPrincipalList, aBlockItems);
   NS_ASSERTION(aBlockItems.IsEmpty(), "What happened?");
   aBlockItems.Clear();
   aNewItems.AddChild(blockFrame);
 }
 
@@ -11355,17 +11365,17 @@ nsCSSFrameConstructor::ProcessChildren(n
     nsBlockFrame* blockFrame = NS_NewBlockFrame(mPresShell, blockSC);
     // We might, in theory, want to set NS_BLOCK_FLOAT_MGR and
     // NS_BLOCK_MARGIN_ROOT, but I think it's a bad idea given that
     // a real block placed here wouldn't get those set on it.
 
     InitAndRestoreFrame(aState, aContent, aFrame, blockFrame, false);
 
     NS_ASSERTION(!blockFrame->HasView(), "need to do view reparenting");
-    ReparentFrames(this, blockFrame, aFrameItems);
+    ReparentFrames(this, blockFrame, aFrameItems, false);
 
     blockFrame->SetInitialChildList(kPrincipalList, aFrameItems);
     NS_ASSERTION(aFrameItems.IsEmpty(), "How did that happen?");
     aFrameItems.Clear();
     aFrameItems.AddChild(blockFrame);
 
     aFrame->AddStateBits(NS_STATE_BOX_WRAPS_KIDS_IN_BLOCK);
   }
@@ -11425,17 +11435,17 @@ nsCSSFrameConstructor::WrapFramesInFirst
     // subsequent children; insert lineFrame into aFrameItems.
     aFrameItems.InsertFrame(nullptr, nullptr, aLineFrame);
 
     NS_ASSERTION(aLineFrame->StyleContext() == firstLineStyle,
                  "Bogus style context on line frame");
   }
 
   // Give the inline frames to the lineFrame <b>after</b> reparenting them
-  ReparentFrames(this, aLineFrame, firstLineChildren);
+  ReparentFrames(this, aLineFrame, firstLineChildren, true);
   if (aLineFrame->PrincipalChildList().IsEmpty() &&
       (aLineFrame->GetStateBits() & NS_FRAME_FIRST_REFLOW)) {
     aLineFrame->SetInitialChildList(kPrincipalList, firstLineChildren);
   } else {
     AppendFrames(aLineFrame, kPrincipalList, firstLineChildren);
   }
 }