--- a/layout/base/GeckoRestyleManager.cpp
+++ b/layout/base/GeckoRestyleManager.cpp
@@ -861,16 +861,22 @@ 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;
   }
+
+  if (!newParentContext && !oldContext->GetParent()) {
+    // No need to do anything here.
+    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.