If they're continually adding features and not breaking backwards compatibility on a "fixed" timeline, then the major version jumps that Firefox, IE do etc. seem more pointless than anything.
If I've got this application that we're continually developing on and we're using a 2 month lifecycle for development ... at what point does it make sense to jump a major version? Never? What's the point of it then?
It's the fact that we've become accustomed to major versions representing periods of time.
Version numbers should be representative. If it was the case that they used the minor number to represent their two month cycles, then they'd be at 1.11. What's the point of the 1 in that scenario? It's redundant.
The major version increments fit in better with their development lifecycle, so why not use it?