Don't see any crashes with Firefox and Flash and I literally use dozens of Flash sites daily, if not more, on FireFox in both Vista and Win 7 without issue.
Are you running on Win XP, or some other out of date buggy OS that should be in mothballs?
Also, are we talking the latest Firefox 12 or some out of date buggy version?
FYI, more often than not, it's all the buggy add-ons that seem to cause problems with Firefox than Firefox itself. I recently dumped a bunch of the more questionably coded add-ons, esp. the SEO stuff as as know how flaky most SEO software is to start with, and it's running much better.