I made some changes as to how packages from the Debian Mozilla team that can’t yet be distributed in the Debian archives are distributed to users. Please update your APT sources and now use the following for 4.0 beta packages:
deb http://mozilla.debian.net/ experimental iceweasel-4.0
You’ll also need the experimental repository in your sources, but the overall installation is much easier now:
# apt-get install -t experimental iceweasel
This should work for squeeze and unstable users.
I also added Iceweasel 3.6 backports for Debian Lenny users. For these, add the following APT source:
deb http://mozilla.debian.net/ lenny-backports iceweasel-3.6
You’ll also need the lenny-backports repository in your sources. As for the experimental packages above, installation should be as easy as:
# apt-get install -t lenny-backports iceweasel
If your APT complains about the archive key, please check the instructions to add the key to your APT keyring.