Useful, fairly small, and EASY Rated 4 out of 5 stars
I will say much the same as the other reviewer... This addon is fairly small, does what it says it does, and does that well (ie, it completely automates the process, so no typing/defining per each profile). It could be smaller size, and restartless, and yes... there are other options out there, but it's easy to use, and useful for those using multiple profiles (especially simultaneously).
[see: http://www.howtogeek.com/howto/internet/firefox/use-multiple-firefox-profiles-at-the-same-time/ ]