I\'m using an 3rd party application for device detection called FiftyOne Degrees (http://51degrees.mobi).
That application uses WURFL (http://wurfl.sourceforge.net/)
I think the easiest way would be to look at the web browsers patch file for wurl. (http://wurfl.sourceforge.net/web_browsers_patch.xml) It contains all the browsers that people usually use. You should have this file in your App_Data folder. You should be able to just update your patch file with a reference to firefox 4, and declare a fallback browser type.
I'm using the user agent string found here: http://hacks.mozilla.org/2010/09/final-user-agent-string-for-firefox-4/
could change to:
I haven't had this issue before, but this is the way I would approach your issue. Hope this helps :)