Quick 'n' Easy
Due to a trademark dispute, Firefox under Debian is re-branded as Iceweasel.
If you try to upgrade to the latest version, Firefox 4, you may have some difficulty.
Here's a fast easy method to install Firefox 4 on Debian:
1. Step One
Add the following entry in /etc/apt/sources.list (or a new file in /etc/apt/sources.list.d/:)
deb http://mozilla.debian.net/ squeeze-backports-4.0 iceweasel
2. Step Two
3. Step Three
Install with apt-get
apt-get install-t squeeze-backports iceweasel
4. Step Four
Launch and enjoy.