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:
@echo off
set MOZ_NO_REMOTE=1
start "" "C:\Program Files\Mozilla Firefox\firefox.exe" -p
set MOZ_NO_REMOTE=0
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.
Have phun! 
