User Agent String.Com 

User Agent String explained :

Firefox 3.6a1pre
Mozilla	It's a Mozilla based browser
5.0	Mozilla Version
X11	X Window System a windowing system for bitmap displays
U	Security values: N for no security U for strong security I for weak security
Linux x86_64	Operating System: Linux running on a Intel CPU running on a 64 bit processor
en-US	Language Tag, indicates the language for which the client had been localized (e.g. menus and buttons in the user interface) en-US = English - United States
rv:1.9.2a1pre	CVS Branch Tag The version of Gecko being used in the browser
Gecko	Gecko engine inside
20090428	Build Date: the date the browser was built
Firefox	Name : Firefox
3.6a1pre	Version
First visit:	2009.04.28 12:24
Last visit:	2010.03.18 10:09
All Firefox user agent strings