I wish there were a simple way to toggle our useragent -- doing it through about:config takes forever.
You can go to about:config, do the tweak, and then it will give you a URL in the address bar; that URL, in fact, is the request that actually made that setting change. So you can bookmark that URL, and have a folder of bookmarks that set the UA. Unfortunately, they do leave you at the about:config page, not back at the page you were on (but reloaded with the new UA), but still a lot quicker.