xMirror v0.1.1 available from Meatme adds 3 extensions mirrors to the Firefox extension dialog.
User Agent Switcher gives the user the ability to change the User Agent string (the string that identifies your browser to the web server at the other end of the pipe) between the default of Firefox, IE, Opera and Netscape. You can edit the existing User Agent strings as well as add and delete strings. The page listed also shows you what your User Agent string is.