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.
Read it here
Firefox 3.1 Beta 1 was released on October 14, 2008 and it currently comes in 36 fully-localised languages. This beta is based on Gecko 1.9.1 layout engine and according to the official website, this release features significant changes to improve web compatibility, performance and ease of use. You can read the release notes on the official website, here.