Quote
hacks.mozilla.org
6) The Gecko build date in the “Gecko/yyyymmdd” token has been frozen to 20100101 for Firefox releases. This means:

* If you want to detect whether the browser is genuine Gecko, search for the strings “Gecko” and “rv:”. The presence of these two strings will distinguish Gecko from browsers who state they are “like Gecko” in the UA string.
* If you want to determine the Gecko version, first — as noted above — consider carefully whether you can feature-sniff instead. If you must, obtain it from the “rv:x.y.z” string.

If you are presently using the build date or otherwise depending on the exact format of the “Gecko/yyyymmdd” token, stop! In the next major release of Gecko, this will be replaced by the string “Gecko/”. If you rely on the fact that this will be followed by a date string, you will break.