diff --git a/layout/base/nsCSSFrameConstructor.cpp b/layout/base/nsCSSFrameConstructor.cpp
index e5e2601a58a5..67fb1b5845cf 100644
--- a/layout/base/nsCSSFrameConstructor.cpp
+++ b/layout/base/nsCSSFrameConstructor.cpp
@@ -1136,12 +1136,18 @@ public:
   ~nsFrameConstructorState();
   
   // Function to push the existing absolute containing block state and
-  // create a new scope.
+  // create a new scope. Code that uses this function should get matching
+  // logic in GetAbsoluteContainingBlock.
   void PushAbsoluteContainingBlock(nsIFrame* aNewAbsoluteContainingBlock,
                                    nsFrameConstructorSaveState& aSaveState);
 
   // Function to push the existing float containing block state and
-  // create a new scope
+  // create a new scope. Code that uses this function should get matching
+  // logic in GetFloatContainingBlock.
+  // Pushing a null float containing block forbids any frames from being
+  // floated until a new float containing block is pushed.
+  // XXX we should get rid of null float containing blocks and teach the
+  // various frame classes to deal with floats instead.
   void PushFloatContainingBlock(nsIFrame* aNewFloatContainingBlock,
                                 nsFrameConstructorSaveState& aSaveState,
                                 PRBool aFirstLetterStyle,
@@ -7391,6 +7397,10 @@ nsCSSFrameConstructor::ConstructMathMLFrame(nsFrameConstructorState& aState,
       return rv;
     }
 
+    // Push a null float containing block to disable floating within mathml
+    nsFrameConstructorSaveState saveState;
+    aState.PushFloatContainingBlock(nsnull, saveState, PR_FALSE, PR_FALSE);
+
     // MathML frames are inline frames, so just process their kids
     nsFrameItems childItems;
     if (!newFrame->IsLeaf()) {
@@ -8345,7 +8355,7 @@ nsCSSFrameConstructor::GetFloatContainingBlock(nsIFrame* aFrame)
   
   // Starting with aFrame, look for a frame that is a float containing block
   for (nsIFrame* containingBlock = aFrame;
-       containingBlock;
+       containingBlock && !containingBlock->IsFrameOfType(nsIFrame::eMathML);
        containingBlock = containingBlock->GetParent()) {
     if (containingBlock->IsFloatContainingBlock()) {
       return containingBlock;
diff --git a/layout/base/nsCSSFrameConstructor.h b/layout/base/nsCSSFrameConstructor.h
index f718d9aa1f0f..e41bc4426e4e 100644
--- a/layout/base/nsCSSFrameConstructor.h
+++ b/layout/base/nsCSSFrameConstructor.h
@@ -686,8 +686,15 @@ private:
 
   nsIFrame* GetFrameFor(nsIContent* aContent);
 
+  /**
+   * These functions are used when we start frame creation from a non-root
+   * element. They should recreate the same state that we would have
+   * arrived at if we had built frames from the root frame to aFrame.
+   * Therefore, any calls to PushFloatContainingBlock and
+   * PushAbsoluteContainingBlock during frame construction should get
+   * corresponding logic in these functions.
+   */
   nsIFrame* GetAbsoluteContainingBlock(nsIFrame* aFrame);
-
   nsIFrame* GetFloatContainingBlock(nsIFrame* aFrame);
 
   nsIContent* PropagateScrollToViewport();
diff --git a/layout/mathml/content/src/mathml.css b/layout/mathml/content/src/mathml.css
index 6e00af68585c..9fdaf2c63c6c 100644
--- a/layout/mathml/content/src/mathml.css
+++ b/layout/mathml/content/src/mathml.css
@@ -42,9 +42,8 @@
 
 @namespace url(http://www.w3.org/1998/Math/MathML);
 
-/* MathML doesn't permit floating and positioning */
+/* MathML doesn't permit positioning */
 *, * > *|* {
-  float: none !important;
   position: static !important;
 }