Multiple Firefox profiles?
If you are developing an extension to change the behavior/look of the Firefox browser, it is clever to isolate your modifications in a different profile during development. Only when you are satisfied with your extension should you return to your default profile and apply your extension.
Setting up a new Firefox profile in Windows 7
Close Firefox. You can't modify the user profiles when Firefox is already running one of them.
Type "run" in the start/search box in the lower-left corner.
Type "firefox.exe -ProfileManager" in the Run tablet.
Create a new user profile. Move the hi-lite bar up and down to set the one you wish to run.
Your new profile will not share the homepage, bookmarks, etc. of your default Firefox profile. I made myself a simple webpage that I could point my development Firefox profile at so as to minimize confusion of which Firefox I was using.