Thanks to Linux, SSH and the X architecture which together make running multiple instances of Firefox in the same screen possible.
Step 1: Start your X and login with an account, start the first Firefox instance.
[dgthanhan@localhost ~]$ firefox&
Step 2: SSH to your own localhost using another account with X tunneled[dgthanhan@localhost ~]$ ssh -X monkeya@localhost
monkeya@localhost's password:
Last login: Sat Jan 13 02:11:45 2007
/usr/bin/xauth: creating new authority file /monkeya/.Xauthority
monkeya@localhost's password:
Last login: Sat Jan 13 02:11:45 2007
/usr/bin/xauth: creating new authority file /monkeya/.Xauthority
Step 3: Start the second Firefox instance by invoking "firefox". Enjoy :)
[monkeya@localhost ~]# firefox&
(Note: This guide is only for *nix user)