wmode="opaque" + Firefox + position:fixed = AAAAGGGHH!!!!

I just spent the last 4 hours or so tracking down this bug. I needed to use wmode="opaque" on my Flex app, due to a dynamic menu overlay issue. However, in Firefox this seemed to mean that none of the buttons in the Flex app would be clickable. Ok, Tom, break the problem down...

I embed the swf in a plain-jane web page, and it works fine (wmode="opaque"). I add in my style sheets, and it stops working. Turns out that setting position:fixed on the body tag was the cause - changing to position:static was the cure.

May you waste less time than I did.