I'm using Firefox 2.0.0.12 with Flash Player 9.0.115.0, on a MacBook running OS X 10.4.11. When I use full screen, FlowPlayer distorts the aspect ratio if it's different from the screen's (8x5 in this case).
For example, the video on the front page (http://flowplayer.org/) is squashed horizontally if I fullscreen it. If I use the Greasemonkey script Youtube Alternate Video Player (http://userscripts.org/scripts/show/13955), YouTube videos are stretched horizontally.