If you’re using MOUSE_LEAVE to check for the user’s mouse leaving your SWF in a browser and it’s not working on Firefox on Windows only — check your Embed code. If your problem occurs only on Firefox on PC (i.e. it works everywhere else) and there’s a WMODE parameter defined in your Embed tag or SWFObject instance, try removing WMODE and see if it fixes the FF issue. Thanks for the tip, Nate!
The downside of this fix, of course, is that it doesn’t work if you need your SWF to have a transparent background.