The current production release of Firefox is version 2, but developer releases of version 3 have been available since November 2007. Today Firefox released the beta 3 version of Firefox 3. You can download it here.
I love version 3 of Firefox since most of the memory leak issues on Macs seem to have been fixed (I added it to my list of top applications in December based on that). But a lot of key plugins don’t work yet on Firefox 3, and a lot of sites break when viewed through it as well. Because of that, I’ve switched back to version 2 for now. A list of known issues is here.
Firefox currently has around 17% market share among browsers, second only to Internet Explorer.