You go to the link you'd normally go to to get the extension, here is the link https://addons.mozilla.org/en-us/firefox/addon/user-agent-switcher/ but go in IE. Then download the extension, it downloads as a file with extension xpi. It doesn't install automatically of course 'cos you're in IE and it is just downloading a file and not opening it / interpreting it. When I try in Chrome for example, I click Download, it says
"To install this add-on and thousands more, get Firefox, a free and open web browser from Mozilla.
Learn more about Firefox
or download anyway"
You choose Download anyway.
The file you'll see, is user_agent_switcher-0.7.3-fx+sm.xpi
Then open firefox, click file, then open, make sure it's on "all files" so you see files of any extension, then open that file. From there it installs automatically, so from there it's just like you'd go gone to the site in firefox and clicked to install the extension. It downloads the extension which is quick and says to restart firefox.
You know this user switcher extension is installed 'cos it's listed in Tools..Addons, and it's made a new option in the Tools menu called "Default user agent"