All I did was go to tools->add-ons->plugins, and disable the Flash 10 plugin. I restarted firefox, and enabled the flash 8 plugin using the same procedure. I'm not sure if a version 9 plugin exists, but if you have it you could try enabling that one.
This is not a bug in Firefox, but a bug with the Adobe Plugin; it is adobe's responsibility, not FFs. I thought it was annoying however since FF insisted originally that I needed to download the plugin to fix "severe security vulnerabilities" I did get it from adobe.com though.
Considering Adobe has yet to even announce anything about a 64-bit plugin for ANY browser, and how slow they are releasing pretty much anything, I think we have a while yet.
After a few days of random restarts FF told me that the Flash 10 plugin was causing the crashes; I rolled back and haven't had a problem since.
Also, I have encountered very few sites that require Flash 10; not that it matters, IMO, it's more important that the browser works; if I couldn't use any flash version I would disable it completely.