--- a/layout/base/GeckoRestyleManager.cpp
+++ b/layout/base/GeckoRestyleManager.cpp
@@ -861,23 +861,16 @@ GeckoRestyleManager::ReparentStyleContex
   nsIFrame* providerChild = nullptr;
   if (isChild) {
     ReparentStyleContext(providerFrame);
     // Get the style context again after ReparentStyleContext() which might have
     // changed it.
     newParentContext = providerFrame->StyleContext();
     providerChild = providerFrame;
   }
-  NS_ASSERTION(newParentContext, "Reparenting something that has no usable"
-               " parent? Shouldn't happen!");
-  // XXX need to do something here to produce the correct style context for
-  // an IB split whose first inline part is inside a first-line frame.
-  // Currently the first IB anonymous block's style context takes the first
-  // part's style context as parent, which is wrong since first-line style
-  // should not apply to the anonymous block.
 
 #ifdef DEBUG
   {
     // Check that our assumption that continuations of the same
     // pseudo-type and with the same style context parent have the
     // same style context is valid before the reresolution.  (We need
     // to check the pseudo-type and style context parent because of
     // :first-letter and :first-line, where we create styled and
@@ -893,16 +886,38 @@ GeckoRestyleManager::ReparentStyleContex
                      nextContinuationContext->GetPseudo() ||
                    oldContext->GetParent() !=
                      nextContinuationContext->GetParent(),
                    "continuations should have the same style context");
     }
   }
 #endif
 
+  if (!newParentContext && !oldContext->GetParent()) {
+    // No need to do anything here.
+#ifdef DEBUG
+    // Make sure we have no children, so we really know there is nothing to do.
+    nsIFrame::ChildListIterator lists(aFrame);
+    for (; !lists.IsDone(); lists.Next()) {
+      MOZ_ASSERT(lists.CurrentList().IsEmpty(),
+                 "Failing to reparent style context for child of "
+                 "non-inheriting anon box");
+    }
+#endif // DEBUG
+    return NS_OK;
+  }
+
+  NS_ASSERTION(newParentContext, "Reparenting something that has no usable"
+               " parent? Shouldn't happen!");
+  // XXX need to do something here to produce the correct style context for
+  // an IB split whose first inline part is inside a first-line frame.
+  // Currently the first IB anonymous block's style context takes the first
+  // part's style context as parent, which is wrong since first-line style
+  // should not apply to the anonymous block.
+
   nsIFrame* prevContinuation =
     GetPrevContinuationWithPossiblySameStyle(aFrame);
   nsStyleContext* prevContinuationContext;
   bool copyFromContinuation =
     prevContinuation &&
     (prevContinuationContext = prevContinuation->StyleContext())
       ->GetPseudo() == oldContext->GetPseudo() &&
      prevContinuationContext->GetParent() == newParentContext;