For some testing I need to run two instance of different firefox versions at the same time. But with default setting it is not possible.
It can be possible with firefox profilemanager
On my Ubuntu box I have two versions of Firefox one is beta another one is stable.
I can run stable version via firefox command, to run beta version I issue firefox-4.0 command.
So the trick is creating a profile for another version firefox, in my case Beta Firefox
To create profile run
firefox-4.0 -P --no-remote
Creating Custom Launcher
Here I am using GNOME. To create custom launcher, right click in application launcher pannel, choose add to panel