Steps to run Firefox 2 and 3 at the same time: (Assuming you have already installed Firefox2)

1. Create a new Firefox profile: In the Windows Run box (Win+R) type:

   firefox -profilemanager -no-remote.

   Click on “Create Profile”, name it “firefox3″.

2. Download a nightly build of Firefox3. Download the zip file (firefox-xxxx.en-US.win32.zip). Extract it in some directory, say c:\myapps\ff3

3. Run Firefox3: In Windows Run Dialogue type:
   c:\myapps\ff3\firefox.exe -P minefield -no-remote

Reference: blog.codefront.net