I'm a Firefox junkie. On this machine I've got 2.0.0.18 that I no longer use, and 3.0.5 which I use sometimes, and Gran Paradiso (the Beta version, now at 3.0.6 Pre) that I use most of the time.
I also had the two "further out" versions that are in test (Minefield and the other one), but after a year of not having the plugins I like, and never finding a bug, I decided to stop being a masochist and removed those.
All the settings I liked on Firefox 2 carried over automatically into 3, and Gran Paradiso, Minefield, etc. If I load a new shortcut into the shortcut toolbar on one version, it's there on all of them. That's clever!
I did download a newly-launched plugin to protect my passwords etc, but all it did was stop automatic logins using passwords, which rather defeated the whole object of storing passwords in Firefox.