Problems with AS2 in Mac Firefox receiving JS mouse events

AS2, Flash CS4. We’re using ExternalInterface to get callbacks from JavaScript for mouseover and mouseout when the user rolls on or off the .swf on a page. This works fine in all situations except for one:

If the .swf is viewed in Mac Firefox, and if there is a button or mc near the edge of the stage with an onRelease() event, when you mouse from outside of the .swf into the button, the mouseover fires correctly; but when you mouse out from the button and continue outside of the .swf, mouseout never fires.

We have built a small, simple testbed with only minimal code and assets (ExternalInterface code and a button). We are able to duplicate this consistently, on multiple machines. (again, Mac Firefox only).

You can view the testbed at:
http://clientdata.teracent.net/mouseout/noswfobject.html

If you have Firebug enabled, you can also view debug output to the console.

Any suggestions appreciated. Thanks!