Sometimes it is needed to run several instances of Mozilla (Firefox/Thunderbird) at the same time, using different profiles.

The MOZ_NO_REMOTE environment variable does the trick.

• start the profile manager:

  bash $ MOZ_NO_REMOTE=1 firefox -ProfileManager
      

• start another instance using a particular profile:

  bash $ MOZ_NO_REMOTE=1 firefox -P <profile name>
      

On Windows, the following script (.cmd or .bat) wraps firefox, allowing to start several instances of this application:

@set MOZ_NO_REMOTE=1
@start /D"c:\Program Files\Mozilla Firefox" firefox %*

The same thing is applicable to thunderbird as well.