diff --git a/layout/generic/nsFrameList.cpp b/layout/generic/nsFrameList.cpp
index 360c5a63e261..f71483924341 100644
--- a/layout/generic/nsFrameList.cpp
+++ b/layout/generic/nsFrameList.cpp
@@ -97,7 +97,10 @@ void
 nsFrameList::RemoveFrame(nsIFrame* aFrame)
 {
   NS_PRECONDITION(aFrame, "null ptr");
+#ifdef DEBUG_FRAME_LIST
+  // ContainsFrame is O(N)
   NS_PRECONDITION(ContainsFrame(aFrame), "wrong list");
+#endif
 
   nsIFrame* nextFrame = aFrame->GetNextSibling();
   if (aFrame == mFirstChild) {
@@ -137,7 +140,9 @@ nsFrameList
 nsFrameList::RemoveFramesAfter(nsIFrame* aAfterFrame)
 {
   NS_PRECONDITION(NotEmpty(), "illegal operation on empty list");
+#ifdef DEBUG_FRAME_LIST
   NS_PRECONDITION(ContainsFrame(aAfterFrame), "wrong list");
+#endif
 
   nsIFrame* tail = aAfterFrame->GetNextSibling();
   // if (!tail) return EmptyList();  -- worth optimizing this case?
@@ -193,8 +198,11 @@ nsFrameList::InsertFrames(nsIFrame* aParent, nsIFrame* aPrevSibling,
   NS_ASSERTION(!aPrevSibling ||
                aPrevSibling->GetParent() == aFrameList.FirstChild()->GetParent(),
                "prev sibling has different parent");
+#ifdef DEBUG_FRAME_LIST
+  // ContainsFrame is O(N)
   NS_ASSERTION(!aPrevSibling || ContainsFrame(aPrevSibling),
                "prev sibling is not on this list");
+#endif
 
   nsIFrame* firstNewFrame = aFrameList.FirstChild();
   nsIFrame* nextSibling;
@@ -213,9 +221,7 @@ nsFrameList::InsertFrames(nsIFrame* aParent, nsIFrame* aPrevSibling,
     mLastChild = lastNewFrame;
   }
 
-#ifdef DEBUG
   VerifyList();
-#endif
 
   aFrameList.Clear();
   return Slice(*this, firstNewFrame, nextSibling);
@@ -604,7 +610,7 @@ nsFrameList::GetNextVisualFor(nsIFrame* aFrame) const
 }
 #endif
 
-#ifdef DEBUG
+#ifdef DEBUG_FRAME_LIST
 void
 nsFrameList::VerifyList() const
 {
diff --git a/layout/generic/nsFrameList.h b/layout/generic/nsFrameList.h
index a9432989eb7d..1d8b5eacbed3 100644
--- a/layout/generic/nsFrameList.h
+++ b/layout/generic/nsFrameList.h
@@ -45,6 +45,9 @@
 
 class nsIFrame;
 
+// Uncomment this to enable expensive frame-list integrity checking
+// #define DEBUG_FRAME_LIST
+
 /**
  * A class for managing a list of frames.
  */
@@ -60,9 +63,7 @@ public:
     mFirstChild(aFirstFrame), mLastChild(aLastFrame)
   {
     MOZ_COUNT_CTOR(nsFrameList);
-#ifdef DEBUG
     VerifyList();
-#endif
   }
 
   nsFrameList(const nsFrameList& aOther) :
@@ -265,9 +266,6 @@ public:
 
 #ifdef DEBUG
   void List(FILE* out) const;
-protected:
-  void VerifyList() const;
-public:
 #endif
 
   static nsresult Init();
@@ -437,6 +435,12 @@ public:
   };
 
 private:
+#ifdef DEBUG_FRAME_LIST
+  void VerifyList() const;
+#else
+  void VerifyList() const {}
+#endif
+
   static const nsFrameList* sEmptyList;
 
 protected: