I myself has been trying firefox 3 beta 1 all the time since it’s released on Nov.19th. As to my experience, memory management has been improved a lot. I did try to keep the browser running the whole day and i did find it never took more than 100MB memory allocation. I’m pretty well satisfied.
Now beta 2 is around the corner. As said, easier to use, faster and less memory usage are the key improving aspects. Wow, just cann’t wait to try it out.
And guess I would not go back to Firefox 2, too, if those of my previously installed add-ons could be upgraded to work under Firefox 3.
Update(Nov 19th,2007): It’s released today. Get it by Help->Check for updates... in Firefox 3 Beta 1.