I've been running Minefield (Firefox 3 beta) for a long time now with very little or no problems, so its not often that I switch back to Firefox 2, but its always handy to have that fall back.
I've heard people say and I've been asked a few times recently that they don't want to try FF3beta because they rely on Firefox 2 and don't want to break it or they are using the portable version because FF2 and FF3 won't run at the same time.
Well actually, neither of those are true. You can run FF2 and FF3 at the same time and you can stop them from breaking one another. What you have to do is use the command line arguments when starting Firefox or Minefield. There's a small bit of jiggery-pokerery to start with but once thats done it runs without a problem.
There are only 2 command line arguments that you really need to worry about for this these are "-ProfileManager", "-P" and "-no-remote".
Firefox has the ability to run with multiple profiles, so you could, for instance, have a profile for when you're doing dev work and a profile for you average everyday surfing. Its this functionality that you need to leverage to allow FF2 and FF3 to run at the same time.
To create a new profile, edit the shortcut for Firefox2, appending "-ProfileManager" to the shortcut target. You will also need to add "-no-remote". This will allow you to run more than one instance of Firefox at a time as long as they are using different profiles. Giving you :
"C:\Program Files\Mozilla Firefox\firefox.exe" -ProfileManager
Now start-up firefox. You'll get a dialogue box that will have a button "Create Profile". Click on this and create a profile! I would recommend, for ease, that you call it something like "minefield" (all in lowercase). While you're here, rename the default profile to "firefox2", no spaces.
Don't bother going into firefox once you're done. Just hit the "exit" button.
Now that you've got profiles for both firefox and minefield you'll need to edit the shortcuts for both firefox and minefield so that when you start them they start using the appropriate profile. For this you need the "-P" argument. On the firefox shortcut, remove the "-ProfileManger" argument and add in "-P firefox2". On the Minefield shortcut you need to add "-no-remote -P minefield". And that should be everything! You should now be able to startup Firefox 2 and Minefield at the same time with different profiles.
One minor note: The reason I used lower case profile names and no spaces is that the profile names are case sensitive and you need to start wrapping the profile names in quotes if you have spaces or other special characters. Lowercased, no spaces, no special characters is just easy.