I find it useful to have multiple Firefox profiles for developing and testing. A clean profile for testing allows you to replicate most users’ en­vi­ron­ments, who don’t install extensions. Running a de­vel­op­ment profile in a separate profile lets you restart the browser without messing with your default en­vi­ron­ment. You can also run Firefox 2 and Firefox 3 side-by-side in separate profiles.

More at the Cozi Tech Blog.