I found a tutorial on how to have both Firefox 2 and 3 running on your computer so we can check site compatibility in both browsers. The original tutorial is located at TroyRutter.com but I made it a little more conducive to my “special” needs below (after failing a few times I noted some things that you could probably use too!). Here’s Troy’s tutorial with a few add-ins of my own:
Profiles
Firefox uses Profiles to contain your favorites and settings. Most people never see these profiles since they just start it up and use the default profile. This is usually adequate for most users but some, like me, need more than one profile.
So, first I had to make a new profile. Since I already had Firefox 3 installed and over-wrote my Firefox 2, I was going to create a new profile and use the new one for Firefox 3.
To create a new profile, I had to exit Firefox and then click on Start > Run and browsed to the location that Firefox 3 was in. Then, I had to type in -profilemanager (or -p) to launch the manager.
So, the run command looked something like:
firefox.exe -P
This brought up a Profile selection screen, where I clicked on Create Profile and created the new profile (i.e. “Default User“) Then I double clicked that new profile, and Firefox3 launched with that profile.
Downloading Firefox 2 Again
The next task was to download Firefox 2, which you can find in the Firefox Downloads.
I then installed it into a separate directory than Firefox 3 by selecting the CUSTOM setup type (versus the Standard default), clicked Next twice, and then chose a new directory like:
C:\Program Files\Mozilla Firefox 2\
After installing, the Firefox icon will give you the OLD Firefox (since it tells Windows that Firefox is now in the Firefox 2 directory.
Create Shortcut
To create shortcut to the Firefox 2 icon (so that you can differentiate between the two), right click on the FF 2 icon (on your desktop or wherever) and choose Properties. Here you can change the Target and Start In locations of the executable. Make it point to your Firefox 2 directory (should be just “C:\Program Files\Mozilla Firefox 2\firefox.exe”). You might also want to rename shortcut to Firefox 2 in the General Tab instead of just plain Firefox.
Specify the Profile
There is one little thing you have to do to the Firefox 2 icon. You need to specify the profile (so that the icon only opens FF 2 and not 3)
Go back to the icon’s properties > Shortcut > Target and add -P profilename -no-remote. The Target should now look like:
“C:\Program Files\Mozilla Firefox 2\firefox.exe” -P profilename -no-remote
In place of profilename I typed default and all of my original add ons loaded into Firefox 2 
Now you should be able to run both Firefox2 and Firefox3 side by side. You can even import your bookmarks if you want.