Howto change the full user-agent in FireFox.
by The Uni-Hacker on Aug.28, 2008, under Misc

Sometimes you’ll run across a free WiFi network that just happens to block the “Gecko” user agent types. In this case, your firefox browser won’t work either and you’ll have to change your Firefox user-agent to something withouth the word “Gecko”.
Type in “about:config” in your location bar and hit enter. Add a string called “general.useragent.override” with a value of “Mozilla”. Or, if you want to stay as close to firefox standards as possible change the string to “Mozilla/5.0 (X11; U; Linux x86_64; en-US; rv:1.9a8) Firefox/2007100619″ which basically makes it look like you are surfing from a Linux computer with Firefox.