This will help you to run different instances of firefox at same time.
You may need to do this if
1) You want to run two Gmail account at the same time using Firefox (or any cookie storing website)
2) You don't want to mix up history/favorite etc of two 'avatars' at same time; i.e 'Work' and 'Personal'
3) You want to test new Firefox 4 compatibility keeping the old Firefox 3 usable
You can run Firefox 3 and Firefox 4 at same time. For that you may want to install Firefox 4 in a separate directory. For example if you already have Firefox 3 installed in C:\Program Files (x86)\Mozilla Firefox directory, install firefox 4 in another folder, say C:\Program Files (x86)\Mozilla Firefox 4
Create a desktop shortcut of Firefox
[Right click on C:\Program Files (x86)\Mozilla Firefox\Firefox.exe and 'Send to' > 'Desktop']
Right click the shortcut and go to 'Properties'
At 'Target' add -ProfileManager.
i.e change to: "C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -ProfileManager
Save the shortcut and double click it. Now Firefox will open with profile manager window
Create a new profile "WorkProfile"
(Remember to uncheck the box which says do not ask next time)
Create again a new desktop shortcut of Firefox. You can name it "Firefox-WorkProfile"
Right click the shortcut -> Properties
At Target add -P WorkProfile -no-remote.
i.e change to: "C:\Program Files (x86)\Mozilla Firefox\firefox.exe" -P WorkProfile -no-remote
Save the shortcut.
Now when you double click it, Firefox will open with new profile WorkProfile
If you need, you can create another profile and all can be run simultaneously
For running firefox 3 and 4, I've created two profiles 'FireFox3' and 'FireFox4' each pointed to the appropriate installation by proper shortcuts.