Well, why would you want to do this? Basically, you can use Firefox instances that are of different versions or maybe even with different profiles.
Open up a .bat file and write this:
start "" "C:\Program Files\Mozilla Firefox\firefox.exe" -p
If you have more than one profile, when you run this .bat file the Profile Manager will pop up, asking you for the profile to load. If not, it will just load up another instance of it.