“Debian Lenny comes with Iceweasel 3.0.3, which is actually Firefox with a changed name and a few minor changes. If you have Iceweasel 3.0.3 installed but want to test this new beta without changing anything to your system, just follow the steps below.
Download Firefox 3.1 Beta 1 Get the Firefox .tar.bz2 archive from here, then uncompress it using:
tar -xjf firefox-3.1b1.tar.bz2
Next, move the firefox directory wherever you want it, for example in your home directory, or in ~/apps etc.”