Debian stable releases come with a re-branded version of Firefox (and a re-branded Thunderbird is available too). However, the version doesn't get upgraded, so Lenny users still have Iceweasel 3.0.6 (unless they've got 3.5.16 from backports) and Squeeze users 3.5.16.
The Debian Mozilla team "provides various versions of some Mozilla related packages for use on different Debian systems". There's a wizard to help you find the packages suitable for your system.
If I'd known about this, I probably wouldn't have installed Firefox on Squeeze- I'd've gone for an updated Iceweasel and saved myself the manual updates (3.5.16 has an annoying bug in the way it handles dark menus in Gnome, which was fixed in 3.6).