To me, the reasoning behind the change in version is pretty clear. Especially looking at the disaster that was IE6. As soon as there is any support for old versions some IT department somewhere will immediately decide it's time to freeze their version. Moving forward after making a freeze like this becomes very difficult (that has been my experience). This presents two options 1) It's their own problem and move on or 2) Continue to patch old versions. The issue with patching old versions is bad IT decisions end up costing Mozilla time. Meanwhile chrome just continues chugging along at full speed only supporting current.
The current linux distro versioning and support structure seems to map well to this problem. Ubuntu, for instance, has LTS versions that IT can stick with and are patched for X many years.
Windows users update their anti-virus software and definitions in near real-time. Operating Systems update near real-time... why not browsers. I now use chrome most of the time and I am (surprisingly) happy with their versioning. I was a naysayer when they first started too.