I'm using an 3rd party application for device detection called FiftyOne Degrees (http://51degrees.mobi).
That application uses WURFL (http://wurfl.sourceforge.net/) to find out what type of device requested the page based on the user agent.
This works great, except for in the new Fire Fox 4. I think this is because FF4 was released on the 22nd March 2011 while the WURFL was last updated on the 6th March 2011.
Has anyone else come across this problem? Any work arounds?