Installing the latest Firefox 3.0 Beta 3 in Ubuntu Gutsy is really simple and you just have to use Synaptic. Just enable the Backport repositorie and install Firefox 3 b3.

System - Administration - Synaptic Package Manager

Settings - Repositories - Updates Tab

Enable “Unsuportted Updates (gutsy-backports)“, close, and press RELOAD

Now, just search for Firefox 3 and install it

You will still have Firefox 2 installed together with FF3