Use Multiple Firefox Profiles at the Same Time

Did you know that you can run more than one Firefox profile at once? This can be extremely useful if you want to test out extensions more easily, create a web development profile, run a slimmed down profile just for Gmail, or if you just want to have a clean profile.
The first thing you’ll need to do is create a new profile. You’ll need to run the following from a run dialog or command prompt, adjusting the path if necessary.

C:\Program Files\Mozilla Firefox\firefox.exe -profilemanager

Create an extra profile by using the Create Profile wizard, and make sure that each profile has a distinct name. You’ll also want to leave the “Don’t ask at startup” box checked.
The next thing you’ll want to do is make another shortcut to Firefox. You can just copy/paste the current shortcut if you feel like it, then open up the properties page and go to the Shortcut tab.

Add the following to the end of the Target box. As you can see, my profile was named Geek, so that’s what I used.

-P PROFILENAME -no-remote

Now on the second shortcut, which I suggest you give a relevant name to, go into the same place except use the profile name for the secondary profile. Make sure you use the -no-remote option!

Now you should be able to use either shortcut to open a new Firefox window specific to that profile. Extensions, bookmarks, themes, will all be specific to the profile, and a new profile will start off at default.
Note: This assumes you are using Firefox 2.0. If you aren’t, then you should be.