Trying to identify component in browser

The other day I typed this into my Firefox (v. 2.0.0.12) browser address bar while helping someone to sort out their computer which had been infected:

javascript:alert(navigator.userAgent)

The response was this:
Mozilla/5.0(Windows; U; Windows NT 5.0; en-US; rv:1.8.1.12) Gecko/20080201 Firefox/2.0.0.12

Most of this I understand but I'm not sure what the U is about and haven't had success in finding out. Any one know?

Thanks!

 

It's an encryption identifier. More info at Wikipedia....
http://en.wikipedia.org/wiki/User_agent