Often we need run a web site or web application via different version of firefox , for example firefox 3.5 and firefox 11 . For this action you can create multi profile for firefox , please follow me to create different profile (in MS Windows) :

1 –  Open CMD : c:\\Program Files\\Mozilla Firefox\\firefox -profilemanager -no-remote

2 – Now Create new profile for firefox 3.5 & 11

3 – After create profiles you most change Target in firefox shotrcut to  :

\"C:\\Program Files (x86)\\Mozilla Firefox\\firefox.exe\" -P FireFox35 -no-remote
Note : FireFox35 is a profile

Note : You most install Firefox 3 and 11 in different places .

Congratulation .