Opening a new instance of your Mozilla application with another profile
This article explains how to open multiple instances of your Mozilla application at the same time, using different profiles. It was written for Firefox but also applies to Thunderbird and SeaMonkey 2. Thunderbird users should read Run multiple copies of Thunderbird at the same time for additional information.
When Firefox is running and you try to open another Firefox process, a new window is opened instead, in the already running process. In Firefox 2 and above, you can use the
-no-remote command line option to start a new Firefox process that is invisible to the first process.  By using
-no-remote along with the
-P option to select a different profile, you can open multiple instances of Firefox at the same time, using separate profiles.  Selecting a different profile from the one used in the first process is necessary because profiles are "locked" when in use.
Important: Never use -no-remote to start the "default" profile (the one that is set to open without asking when you launch Firefox). That's because when you launch Firefox, for example, by clicking a link in your mail program, you will get a Firefox is already running but is not responding message if the default profile is already in use. The solution is to always start the default profile normally and launch all others with -no-remote. Then it will work. 
You can use a shortcut to a specific profile to open Firefox using
-no-remote. For example, to open a profile named "My Profile" using -no-remote, enter the following in the Target field of a Windows desktop shortcut (adjust the path, if different):
"C:\Program Files\Mozilla Firefox\firefox.exe" -P "My Profile" -no-remote
Note: You can also open the Profile Manager with Firefox already running, by using
-P -no-remote without including a profile name.