It is also for marketing. Firefox 6 and 7 will be out in a matter of weeks. There are noticeable updates to them, but nothing that they haven't done before with smaller version numbers, rather than, what is easier to say, release candidate numbers (1,2,3,4,5,6,7 etc). It's because Safari is on version 5, Opera is on version 11, Chrome is on 13 (14 not far off, right?), IE is on version 9 (with 10 around the corner). It's all marketing because people see a version number, the higher it is, they think "oh it must be better".
The only browser I've seen released periodically, with version numbers increased with actual HUGE changes and new features [big ones, like extensions/turbo/speed dial (with live extensions etc)], is Opera.