You will find that a handful of sites and many extensions don't work properly yet, and although that's mostly a non-issue, you'll undoubtedly need a working browser at some point. Sadly, you won't be able to use your preexisting version of Firefox (presumably 3.6.x) without closing the beta first, and that can be annoying.
Of course, you could just hop on Internet Explorer, but what orange-blooded Firefox advocate would stoop to such a level? Chrome and Opera are other obvious choices, but if you must have Firefox, there's a very basic workaround that will let you run multiple instances and versions of Mozilla's browser simultaneously. We are using v3.6.8 and the newly released Firefox 4 Beta 2 for testing.
After installing the beta build, you'll notice that all your data and settings from the previous version such as bookmarks and credentials are automatically loaded, and therein lies the problem: both versions of Firefox are using the same profile. All you have to do is set a specific profile for each build and you are golden.
For the sake of easy access, create a shortcut on your desktop for both Firefox 3.6.x and Firefox 4. Open Run (Windows + R keys) and then paste the following text, changing the file location if needed: C:\Program Files (x86)\Mozilla Firefox\firefox.exe -p -no-remote
That should open the profile manager upon execution, and you'll probably only see one profile named "default." All you have to do is create a new profile for Firefox 4, but you may want to rename your existing profile to keep things tidy. I chose "Firefox3" and "Firefox4," ideally use a name with no space in it.
Once both profiles are in place, right-click each of your Firefox desktop shortcuts, open Properties, head to the Shortcut tab and add this to the end of the Target text: -p PROFILE -no-remote
By creating a fresh profile, Firefox 4 won't have all the user data you've accumulated in Firefox 3, but there is a quick fix for this, too. Go to the Windows directory %appdata%\Roaming\Mozilla\Firefox\Profiles and copy all the contents from your original profile to Firefox 4, overwriting everything.
At this point, both instances of Firefox should be able to run simultaneously and each should have your history, login and other information. If you're still looking for a few things to tweak, be sure to check out our compendium of Firefox 4 Beta tweaks and don't hesitate to share your own.