Firefox/Flash player problem...

Whenever i watch a Flash movie in Firefox, it just shows a white box. I have to scroll down until part of the movie is off screen before it appears, scroll back up, it disappears again. I can still watch the movie as it only needs to be 1 line of pixels that I can't see, but it's still annoying and shouldn't happen

Any ideas on what could be causing this?

I thought I'd check it out in Safari, and it worked fine.