Out of the box, there's no way to make two different version of Firefox, or two copies running different user profiles, run at the same time. This can be a big pain when doing stuff like developing extensions or testing alpha builds, or maybe you just want to have both your and your roommate's profiles open at once. Of course, there's a workaround. It requires setting the MOZ_NO_REMOTE environment variable to 1 and David Schontzler at planet::dojo describes the process. He provides a shell script that will save you a step (choosing a profile in the profile manager) on Mac OS X or Linux, and in the comments another reader provides a Windows VBScript. If you'd prefer a batch file take a look at this tip by "The Edmeister.