deb http://ppa.launchpad.net/fta/ppa/ubuntu jaunty main
click on Close, open a terminal and type: sudo apt-get update
Then, go to System > Administration > Synaptic Package Manager and search for Firefox, select version 3.5 to be installed (it will also install additional files). After install is complete, you can find Firefox 3.5 beta4 in Applications > Internet under the name: Shiretoko Web Browser.
If you would like to add a PPA key (this is so that you won't get any warnings on your Ubuntu, that the repo is not signed):
Step 1: Visit the PPA's overview page in Launchpad.
Step 2: Click the key fingerprint on the overview page. It'll look something like this: B0BE17C2A0C914F086B7B8327D2C7A23BF810CD5`.
Step 3: This will take you to the key's page in the Ubuntu keyserver. Click the ID, which looks something like this: BF810CD5
Step 4: Copy the public key, which will be something like: -- i.e. the portion starting:
-----BEGIN PGP PUBLIC KEY BLOCK-----Version: SKS 1.0.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wb9W-----END PGP PUBLIC KEY BLOCK-----
I'm going to give you the direct link to the FTA public key: here
Step 5: Paste the public key into a text editor and save it.
Step 6: Open System->Administration->Software Sources and click the Authentication tab.
Step 7: Click Import Key File, select the key you saved earlier and you're done!
Please note that if you install Firefox 3.5 beta4, your Firefox 3.0.x will not be installed! Both will work ok, just don't open both versions of Firefox in the same time.