To install muliple versions of Firefox and run them both at the same time, following these instructions:

1. Download both the versions of Mozila Firefox (1.5/2.0) into a seperate directory
2. Install both the versions into a seperate directories using the “Custom” option.
   
   For example, install Firefox 2 in the following directory
   C:\Program Files\Mozilla Firefox2
   
   For example, install Firefox 3 in the following directory
   C:\Program Files\Mozilla Firefox3
3. After installing, open the profile manager. Go to Start > Run and paste the below code
   
   "C:\Program Files\Mozilla Firefox3" -ProfileManager
4. Create two new profiles: "Firefox2" and "Firefox3".
5. Start notepad and paste the following code for FF2.0
   
   @echo off
   set MOZ_NO_REMOTE=1
   start "" "C:\Program Files\Mozilla\Firefox2\firefox.exe" -P "Firefox2.0.x"
   set MOZ_NO_REMOTE=0
   
   Save file as "firefox3.bat” to your desktop
6. Create another file for Firefox 3 in notepad with the following code:
   
   

   @echo off
   set MOZ_NO_REMOTE=1
   start "" "C:\Program Files\Mozilla\Firefox3\firefox.exe" -P "Firefox1.5.x"
   set MOZ_NO_REMOTE=0

   Save it as "firefox3.bat" to your desktop.

7. Execute one or both files.