--- a/layout/generic/nsLineBox.cpp
+++ b/layout/generic/nsLineBox.cpp
@@ -391,22 +391,24 @@ nsLineBox::DeleteLineList(nsPresContext*
   // frame tree while we're destroying.
   while (!aLines.empty()) {
     nsLineBox* line = aLines.front();
     if (MOZ_UNLIKELY(line->mFlags.mHasHashedFrames)) {
       line->SwitchToCounter();  // Avoid expensive has table removals.
     }
     while (line->GetChildCount() > 0) {
       nsIFrame* child = aFrames->RemoveFirstChild();
-      MOZ_ASSERT(child == line->mFirstChild, "Lines out of sync");
+      MOZ_DIAGNOSTIC_ASSERT(child->PresContext() == aPresContext);
+      MOZ_DIAGNOSTIC_ASSERT(child == line->mFirstChild, "Lines out of sync");
       line->mFirstChild = aFrames->FirstChild();
       line->NoteFrameRemoved(child);
       child->DestroyFrom(aDestructRoot, aPostDestroyData);
     }
-
+    MOZ_DIAGNOSTIC_ASSERT(line == aLines.front(),
+                          "destroying child frames messed up our lines!");
     aLines.pop_front();
     line->Destroy(shell);
   }
 }
 
 bool
 nsLineBox::RFindLineContaining(nsIFrame* aFrame,
                                const nsLineList::iterator& aBegin,