Starting with Firefox 5, the Gecko (rendering engine) version numbering will now be the same as Firefox version. Presently with Firefox 4, the Gecko version is 2.0 (though it was 1.9.3 during the Alpha releases as Firefox 3.7). Now with Firefox 5, Gecko version will also be 5. I first noticed this when I was looking at the build id information for the Firefox 6 Nightly build I am currently using.
Mozilla/5.0 (Windows NT 6.1; Win64; x64; rv:6.0a1) Gecko/20110415 Firefox/6.0a1 ID:20110415030205
The rv:6.0a1 portion refers to the Gecko version, which in this case is 6.0 Alpha 1. Compare this to the build id for Firefox 4:
Mozilla/5.0 (Windows NT 6.1; WOW64; rv:2.0) Gecko/20100101 Firefox/4.0 ID:20110318052756
So Mozilla is going to jump from version 2.0 to version 5.0 now. I suppose they are trying to simplify the version numbering as they are with Firefox as well. But still a major change in the numbering use to signify a major change in Gecko. Such as going from Firefox 2 (Gecko 1.8.1) to Firefox 3 (Gecko 1.9) and then Firefox 4 (Gecko 2.0). But, also keep in mind with Mozilla’s new release plan, a new Firefox release ending with .0 is not going to be as significant as it use to be. Under the old system/numbering Firefox 5 would have likely been Firefox 4.1 or may be 4.5.
Of course Mozilla has already greatly reduced the version numbering of future Firefox releases. For example the next update for Firefox 4 is not going to be branded as Firefox 4.0.1 but rather Firefox 4 Macaw (see this post for more information about the code names). Note: The Beta release of Firefox 4.0.1 on April 15th is being labeled as Firefox 4.0.1 Beta (Build 1), but I suspect this will be re-branded as Macaw when it is released on April 26th. So I suppose it would make sense for them to do the same with Gecko. All I know is this new release schedule and version numbering/naming scheme is going to take a little getting used to.