If you point your Firebird browser at about:config (instead of http://whatever.com), it gives you access to your user.js file, which is where you can override your useragent string (among other things).
If you Control-click, it will allow you to add a new string. Name the string general.useragent.override. Give it a value equal to anything in this list. Firebird is now masquerading as your new user agent.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20031205072548753