I cannot say that I've noticed the crashing with Flash so far.
I have been using seamonkey for ages, and it actually seems to me that I have less problems with flash in ff3, compared to seamonkey.
I've recently upgraded the flash plugin to 9.0.124, taking libflashplayer.so from the site.
Maybe upgrading could minimise those problems?
I haven't done any extensive testing, though. I've gone to several flash sites, but have only been using ff3 for 2 days.