The Debian Mozilla Team provides various versions of some Mozilla related packages for use on different Debian systems. Their homepage serves as an excellent basis for adding their repositories to /etc/apt/sources.list, and for installing different versions of Iceweasel (Firefox) and Icedove (Thunderbird). So, I do not repeat it here. However, in order to add their archive key to the apt keyring, one has to install debian-keyring first.
#aptitude install debian-keyring
Then you can successfully follow the instruction on http://mozilla.debian.net/.