I think Firefox is putting out automatic updates too fast and releasing buggy versions myself.
This is an odd one that I have not been able to figure out a "good" solution for, but with some experimenting I found that by adding this in the css:
.fileupload embed { background-color:inherit !important; padding:9px 9px 9px 9px; }
the button is partially visible but still has a white line through it and for it to work you have to click right on the white line part of the button because that is the actual flash object. It just seems the flash object itself is white in Firefox obscuring the button which should show through it like in other browsers.
Wish I had a better solution but at the moment I don't and the above fix doesn't seem to have any negative side effects in other browsers.
Best,
Joe