There are some tips out there on installing Firefox 3 after you have Firefox 2 running.  The following explains how to install Firefox 2 after you have Firefox 3 installed.

1. Install Firefox 3 if you have not already

2. Grab the Firefox 2 portable install.  This will allow you to run Firefox on a USB drive (http://portableapps.com/apps/internet/firefox_portable)

3. Close any Firefox instances you have running

4. Extract Firefox 2 portable to where you want it to reside.  Mine is on my external USB drive

5. Create a new shortcut to the Firefox 2 portable exe.  Modify the Properties on this shortcut so that the target looks like this:

FirefoxPortable.exe  -P FF2 -no-remote

6. Go to start->run and type firefox.exe -ProfileManager

7. Create a new profile called FF2 that will be used for Firefox 2.  I also have mine setup to always prompt me for what profile to use.  I have one for Web development, penetration testing, and research.

Now just execute the shortcut you created for Firefox 2 and you are able to run Firefox 2 alongside Firefox 3.  This is good for testing web applications and running addons that only work on Firefox 2.