Did you install it from the Intrepid or Jaunty repositories? If so, do you realize that you upgraded libglib2.0-0 and libgtk2.0-0 too?
While I agree that recommending Ubuntu binaries is foolhardy let's at least be accurate when we make statements like this
Ubuntu - firefox depends:
http://packages.ubuntu.com/jaunty/firefox-3.0
libgtk2.0-0 (>= 2.16.0) ((>= 2.14.1 intrepid)
libglib2.0-0 (>= 2.16.0) (also intrepid)
Versions in Debian:
libgtk2.0-0 = 2.12.12-1~lenny1 (lenny), 2.16.1-2 (squeeze and sid)
libglib2.0-0 = 2.16.6-1+lenny1 (lenny), 2.20.0-2 (squeeze), 2.20.1-2 (sid)
So libglib2.0-0 is OK , libgtk2.0-0 is only a problem if installing into stable. I didn't check all the other dependencies though. In any case blindly loading Ubuntu binaries means that when (not if) you break your system you get to keep all the pieces. The key word there is blindly, it can work - if you're careful about which ones. But you're still better off grabbing source only and compiling instead.