Firefox isn't bloated in terms of memory consumption, when compared to Chrome. It's interface has more nice features though, which I greatly enjoy though.
Did you actually test a recent version?
Oh btw. the Beta version of Firefox is an officially supported one, meaning if you find something that is bad for security bug you will receive the grant and stuff.
It still looks incredibly ugly on the Mac. The last time I used Firefox on Mac it was wobbly. I only now have it installed for testing. The change in versioning was a mistake which caused a load of confusion. Firefox may be getting better in performance or memory usage but does it matter? Other browsers are already there.
I will keep tabs on Firefox but I see no reason to switch from Safari or anything else for that matter to Firefox at the moment. It is a bit like IE tbh. It is getting better but for a generation of users the reputation damage is already done.