If you update your ubuntu box, then you are now using firefox 3.0.1.
The reason I’m mentioning this, is that some people have reported firefox 3.0.1 didn’t crash anymore when viewing flash videos.
I can’t conform this because I don’t use pulseaudio and libflashsupport.
Edit: Nope, it still crashes.
Switch from PulseAudio to ALSA and remove libflashsupport, that should fix your crashing problems.
Also having an add-on like “flashblock” doesn’t hurt.