Your Firefox User Agent or build ID contains a lot of information about your browser as well as your operating system. Here is break down on what is in your User Agent:

Mozilla/MozVer (Platform; Security; SubPlatform; Language; rv:Revision[; Extension]*) Gecko/GeckVer [Product/ProdVer]

• Mozilla/MozVer ~ All Firefox build IDs start with Mozilla/5.0. Simply put this means the browser meets the standards to be Netscape 5 compatible. This was the first Mozilla based (never released) version of Netscape.
• Platform ~ This is the operating system (but not the version). Possible values include:
  • Windows
  • Macintosh
  • X11 (Linux)
• Security ~ This is the encryption strength of the browser:
  • U: Strong Security, 128-bit encryption (USA)
  • I: Weak Security, 40-bit encryption (International)
  • N: No Security, No encryption
• SubPlatform ~ This is the version of the operating system.
  • Windows:
    • Win 3.11 – Windows 3.11
    • Win 95 – Windows 95
    • Win 98 – Windows 98
    • Win 9X – Windows ME
    • Win NT 4.0 – Windows NT 4.0
    • Win NT 5.0 – Windows 2000
    • Win NT 5.1 – Windows XP
    • Win NT 5.2 – Windows Server 2003 and XP Pro x64
    • Win NT 6.0 – Windows Vista
  • x11 Linux:
    • Linux [Version] Hardware
    • FreeBSD Hardware
    • NetBSD Hardware
    • SunOS sun4u
  • Macintosh:
    • PPC Mac OS X
    • PPC Mac OS X Mach-O
    • Intel Mac OS X
• Language ~ standardized language identifier, Firefox is available in several different languages. The two common English language builds are:
  • en-US = English United States.
  • en-GB =English Great Britain.
• rv:Revision ~ This is the version of Gecko, the browser engine.
• Gecko/GeckoVer ~ This is the date in YYYYMMDD format the revision (or version) of Gecko was was compiled, not necessarily the date it was released
• Product/ProdVer ~ The name and version of the product. Some of these include:
  • Deer Park/1.5
  • BonEcho/2.0.0.1
  • Firefox/1.5.0.9
  • Firefox/2.0.0.1
  • Gran Paradiso/3.0a1
  • Minefield/3.0a2

Below is the user agent as found via Help, About Mozilla Firefox for my current version of Firefox:

Mozilla/5.0 (Windows; U; Windows NT 5.1; en-US; rv:1.8.1.2pre) Gecko/20061223 BonEcho/2.0.0.2pre

I am running a Mozilla product compilable with Netscape 5.0, on a Windows platform, with high 128-bit encryption USA security, on the Windows NT 5.1 (XP) subplatform, using an english-US language. My current build of Firefox is based on the 1.8.1.2pre revision of Gecko which was compiled on 12/23/2006. The product name is BonEcho (nightly build) and the version I am running is 2.0.0.2pre.

Now you can see how valuable your user agent/build ID is for troubleshooting, especially when people are not even sure what operating system they are using much less which version of Firefox.

Source: Wikipedia