While it does switch the User Agent, this addon is pretty much useless when testing Mobile webpages. It doesn't detect the size of the screen to switch the view between profile and landscape. Chrome is far superior with it's built in features to switch User Agents. I prefer Firebug in Firefox over Chrome, but testing mobile sites on Firefox is still way behind the game.
My focus until now has been on my other extension, Web Developer. I do hope to work on User Agent Switcher, but it will require essentially being rewritten from scratch so I’m not sure when I am going to have the time to work on it honestly.