When I ran Gutsy Gibbon, I had the same problem.
Firefox 2 or 3, it didn't matter. Reinstalling flash_nonfree didn't help either.
What I did notice is that when I removed PulseAudio and went back to
Esound again, the problem vanished.
Now I run Hardy Heron, which uses PulseAudio by default, and the
problem is back again. About a quarter of the attempts to play any
Flash movie with sound results in a Firefox crash. These are mostly
YouTube movies.
When I ran Gutsy Gibbon, I had the same problem.
Firefox 2 or 3, it didn't matter. Reinstalling flash_nonfree didn't help either.
What I did notice is that when I removed PulseAudio and went back to
Esound again, the problem vanished.
Now I run Hardy Heron, which uses PulseAudio by default, and the
problem is back again. About a quarter of the attempts to play any
Flash movie with sound results in a Firefox crash. These are mostly
YouTube movies.
Hope that helps a little.