diff --git a/mozilla/layout/generic/nsFrameSetFrame.cpp b/mozilla/layout/generic/nsFrameSetFrame.cpp
index 0e73a2a9531..01ce6f7fb24 100644
--- a/mozilla/layout/generic/nsFrameSetFrame.cpp
+++ b/mozilla/layout/generic/nsFrameSetFrame.cpp
@@ -1009,13 +1009,19 @@ nsHTMLFramesetFrame::Reflow(nsPresContext*          aPresContext,
   height -= (mNumRows - 1) * borderWidth;
   if (height < 0) height = 0;
 
+  nsCOMPtr<nsIFrameSetElement> ourContent(do_QueryInterface(mContent));
+  NS_ASSERTION(ourContent, "Someone gave us a broken frameset element!");
+  const nsFramesetSpec* rowSpecs = nsnull;
+  const nsFramesetSpec* colSpecs = nsnull;
+  PRInt32 rows = 0;
+  PRInt32 cols = 0;
+  ourContent->GetRowSpec(&rows, &rowSpecs);
+  ourContent->GetColSpec(&cols, &colSpecs);
+  // If the number of cols or rows has changed, the frame for the frameset
+  // will be re-created.
+  NS_ENSURE_STATE(mNumRows == rows && mNumCols == cols);
+
   if (!mDrag.mActive) {
-    nsCOMPtr<nsIFrameSetElement> ourContent(do_QueryInterface(mContent));
-    NS_ASSERTION(ourContent, "Someone gave us a broken frameset element!");
-    const nsFramesetSpec* rowSpecs = nsnull;
-    const nsFramesetSpec* colSpecs = nsnull;
-    ourContent->GetRowSpec(&mNumRows, &rowSpecs);
-    ourContent->GetColSpec(&mNumCols, &colSpecs);
     CalculateRowCol(aPresContext, width, mNumCols, colSpecs, mColSizes);
     CalculateRowCol(aPresContext, height, mNumRows, rowSpecs, mRowSizes);
   }