I've been running FF3 since the first public beta. Loved it. I've also been using my typical extensions (AB+, Filterset.G Updater, Firebug, WebDev Toolbar).
The betas used to really crash on me all the time if I opened too many tabs, or a specific tab had some hearty JS or client-side code to compile/run. Seems like they fixed that issue.
Unfortunately, there is already a security vulnerability; although, you have to be stupid enough to click on the link.
Now... if only Thunderbird could get going on a faster release cycle....