This used to be in Tools --> User Agent Switcher, but it doesn't seem to be there now. I also installed another addon and this isn't appearing in the menu either. Can I access Add-Ons from a different location in Firefox 4?
To see the User Agent Switcher options again, you just have to turn on the Menu Bar, which is turned off by default in Firefox 4.
Firefox Button In Top Left --> Options --> Menu Bar