I decided to just update from ver. 6 because of the promised speed and memory usage improvements for 7.
It is noticeably faster, and the memory usage is way, way better! I don't seem to be getting any leaks after an hour of browsing (Win 7 Home Premium), and when I start it it only takes about a third the RAM of six, and when I stop it, all the used memory goes away! Version 6 was bad as I had to restart FF all the time because it would take up close to 2 gigs of RAM after a while, and would only release about 2 - 300 mb when exited.
Now, there's something that you sacrifice when using beta, namely, most your addons don't work FFS!
I'm not happy about Firebug at all, but adblock, and plus still work. I don't want to go back to six now, but if Firebug doesn't get updated soon.... Shite! Almost no developer tools work.
Oh well, I'll just use Chrome or Opera, LOL.
FF 7 is fast, and no longer a mem hog.