Same problem here in the latest version of Flowplayer.

The video shows as a narrow slice, squeezed vertically down to 5 or so pixels. It appears about halfway up the screen, but it's vertical position is different for different occurrances. I'm using the latest version of Firefox on OS X Leopard. This happens sporadically, something like every 1 in 20 - 30 times a page is loaded. Refreshing the page usually causes it to load correctly.