A relatively stable and very attractive beta version of Firefox 3 has been around for a week or two. Windows, Mac, and Linux versions, in most languages you can think of, available free here.
It is a beta -- in fact, beta 4 -- so it has some rough edges. For me the main one is that some Firefox plug-ins I use all the time aren't yet compatible with this version. (Biggest stopper for me: Chinese Perapera-kun, for help reading Chinese characters on line.*) But it has a variety of tricks to make it run faster, and use less memory, than Firefox 2 does. Also, you can keep both 2 and 3 on your machine and use them alternately, though there can be slight start-up delays when you load one after using the other. I switch back to 2 only when I want to use that Chinese plug-in.
F3 B4 has many other nice functional and stylistic improvements, especially in managing downloads and bookmarks and in auto-completing addresses you have typed before. Hasn't crashed or hung for me on either a Mac or PC... yet. Take a look.
(*Update: This wonderful tip from Lifehacker explains how to get plug-ins like Perapera-kun to run under the new beta. It worked! As the Lifehacker post explains, this is in the realm of do-it-yourself surgery and not worth trying if you're squeamish. But it took less than a minute, and it means I can run this new beta version all the time now.)