I had firefox crashes with the 3.0 series. I've been running the Beta since 3.1b3 (now 3.5b4)
Firefox Beta Download. Since the beta, I haven't had a crash. Pages that normally caused problems included a lot of javascript, dhtml, iframes & javascript or other issues. About the only thing that does happen once in a while is that some page with flash will have a banner or video that firefox deems is taking too much time, and it'll prompt me to disable that flash movie. 3.1/3.5 uses a different javascript engine -- I believe the same one chrome uses.
I do use Safari about 30% of the time and while I do prefer parts of it, the firefox beta seems quicker to me.
Firefox beta DOES render things a little differently than you would expect. I've found more code errors in a few sites I've worked with as a result. Most of the coding issues are related to improper nesting of html tags and pages that have too many errors to validate. If you use a lot of addons, running the beta will surely be frustrating as very few of the addons work with it yet.