Want a way to try out the latest Firefox 4 (currently beta 5 - released today) without making any changes to your system? You should try FoxTester, a Firefox extension which you can use with the Firefox version from the official Ubuntu repositories (or any other version), that you can use to install and launch multiple Firefox versions without affecting your default Firefox user profile.
If you do want to permanently use Firefox 4, you can still make it default using FoxTester so that works too!
UsageUsing FoxTester is pretty easy. Just install the extension and once you restart Firefox, a dialog will pop-up - here you have to set the default watch folder. The watch folder can be any folder on your system where you must download the Firefox tar.bz2 files you want to test - like for instance the latest Firefox 4.0 beta 5. You can leave the rest of the settings to their default values.
Once you've downloaded some Firefox version you want to test from Mozilla's website (make sure to place the downloaded archive into the watch folder like we told you above), simply right click anywhere on a webpage to bring up the context menu and select FoxTester > Install and select the Firefox version you want to install. Once installed, right click anywhere again and select FoxTester > Launch and again select the Firefox version you want to launch. In the same way you can also make a Firefox version default and so on.
Here is a video with instructions on using FoxTester, created by lovinglinux, its developer:
For more info and feedback, visit the FoxTester thread @ UbuntuForums.