I'm still using 3.6.17, I just got it from another Vista install of mine and copied over my AppData and it's exactly the same as before, extensions, favorites, addons, cookies and all.
I tried some 3.6 version from some old versions website and it was really buggy. Like, my Gmail didn't even have the option to let me log out or change accounts. Talk about screwy...
But yeah, the new Firefox looks too much like Chrome to me. I like my browser just how it is right now.