I would use 4.0 Beta full-time if Firebug and Yahoo's Delicious extensions were updated. The memory utilization is great just like Firefox 3.6. Compare sometime the memory utilized while running many tabs in Chrome vs. Firefox over a prolonged period of time.
I've found the same, on linux x64 at least. Firefox is a little more stable when you run an ungodly amount of tabs and leave it open for days on end (yes I'm in therapy but so far it's not working). That, plus its superior Vim plugin seals the deal. Looking forward to 4.0, though I'm actually glad they decided to push back the release a few months. Stability > everything, imho.
Update: I found Firebug has an alpha release supporting Firefox 4. http://getfirebug.com/releases/firebug/1.7X/