In case any users out there use Firefox 3.5 series on Linux and have found that when you try to run a video full screen, Firefox crashes, here is why.  It looks like Adobe does not play well with the OpenGL (GLX) implementation for most graphics cards.  Firefox/Flash does not preload the GLX extensions properly so it causes a crash when Flash attempts to load the GLX interface to run fullscreen.  To solve this, create a text file with the following contents:
#!/bin/sh
## replace firefox-3.6 with what you use, e.g. firefox, firefox-3.5, swiftfox
LD_PRELOAD=/usr/lib/libGL.so.1 firefox

Name it something like firefox_preloader.sh and make it executable.  You can either point your shortcuts to it, or copy it to your /usr/local/bin or /usr/bin and use it launch Firefox.  It is a minor kludge, but it works.  This problem is reported on Nvidia and ATI cards.  (I personally like Nvidia cards.)