This is an annoying nvidia bug with the latest Firefox (Shiretoko) version 3.5.
To fix it you need to load the flash plugin before firefox.
To do this you have to find the firefox startup script (mine is in /usr/lib/firefox-3.5.1/firefox.sh) and add the following line at the beginning of it (be careful to not wipe the “#!/bin/sh” line!):
export LD_PRELOAD=/usr/lib/libGL.so.1 Now you can have full-screen videos again in youtube ;) Via:http://ubuntuforums.org/showpost.php?p=7487421&postcount=14