Today I downloaded and installed Firefox 3 Beta 4. I could not do it before, but now that the Web Developer tools are updated and Firebug has a 1.1 beta that works in FF3, that's enough for me.
I don't know about you, but on both Macs I use regularly, Firefox 2 was crashing all the time. Last night, while writing a blog post for BlogHer, my browser crashed at least a dozen times. On my Mac Pro, Firefox completely melted down -- twice -- requiring complete rebuild from the start, manually adding one plug-in at a time. But I had to stick it out because I need those developer tools. I cannot imagine working without Firebug.
The new UI is clean, and seems to take up a bit less space. And so far FF3 is fast. Me likes.