Works but has a few bugs Rated 3 out of 5 stars
When it's working it works well but there are some bugs with it. Your user agent will get reset to default after every restart unless you add an entry to about:config. Create a new boolean entry 'useragentswitcher.reset.onclose' and set it to false. That will fix the reset problem.
Then there's the issue of Agent Switcher not working at all sometimes. For some reason Agent Switcher won't work on my main profile for FF 11. It installs but the user agent list is blank and nothing happens when I import a list and creating a new user agent doesn't work either. I had to create a new profile for it to work. On FF Nightly (FF 13) I never had any problems. Strange.
The UA list that comes with the addon is pretty weak. I don't know why the developer doesn't include a full updated list. It wouldn't be hard to do. I went to http://techpatterns.com/forums/about304.html to download a fairly recent updated list. So this addon can work but be prepared to tweak it if necessary.