text: what is user agent switcher in firefox? ^^^^ "user agent" is a technical information that your browser sends to the web server when you browser a web page. by using that information, the web server knows what browser you're using.  if you switch your user agent, say, to ie, then firefox will pretend to be ie, and the web server will think you're using ie. that's what it does.  it may be useful to view some "ie only" web sites in firefox