With Firefox's release scheme, there's a nice side effect if you use Linux.
Given there is zero support for older versions (except ESR versions such as Firefox 10) distros such as ubuntu and mint are forced to upgrade the browser for security. (btw it's up to the distro to upgrade all your software for fixes, so FF never nags you)
So you're not stuck for years with the same 3.0 or 3.6 while the rest of the world moves on.
Using 3.0.x (while x building up to a big number) for years on Ubuntu 8.04 was not particulary pleasant, esp. at the end.
or Debian Squeeze with its Firefox 3.5, in the day it was fashionable to cling to 3.6 instead of using Firefox 4 then 5 and 6..
I was like, "crap, this old bug with middle mouse scrolling again", suffering the slowness and crashes.
On debian stable, you can use Firefox 10 ESR (it's at some version like 10.0.0.8) after adding a backports repository. For Windows there's probably a setup.exe to download somewhere.
Firefox's scheme is not that bad, considering the browser only gets faster and more stable, and the UI never changes (it's even 95% the same as in version 3.0 if you move the buttons around)