Debian likes to make things difficult. It insists on using an old version of GTK, which means installing Firefox 3 wont be straightforward.
Read it here
This article describes one way to do it: installing GTK and Firefox as a non-root user. I dont understand some of the voodoo behind it, but it works.
However, the default GTK theme is absolutely hideous. I mean Windows 3.1 hideous. Heres some extra steps to fix this: