From a personal marketing perspective i guess they do this version rush because the dummy "consumer" looks at a big number for comparison. Chrome started pulling big numbers to surpass IE, and now firefox is doing the same.
First off, Google rarely even mentions Chrome version numbers in its marketing material, and neither will Mozilla. The http://blog.mozilla.com/ post announcing Firefox 5 does not mention the version number, and neither does the main web site at http://firefox.com/ - also see Chrome's release announcements at http://chrome.blogspot.com/
We actually care much more about delivering features and performance improvements in a timely way than about having a big number before the dot. "Just add one" is a simple rule that saves us from arguing about versions and lets us spend that time improving the browser instead.
Non-technical users rarely even know about version numbers. Meanwhile, people who have any experience in software know that version numbers are and have always been arbitrary. Neither group would be "fooled" by a higher version number, so why would we bother trying?
(Did MacOS really have "more" changes from 8 to 9 than from 10.3 to 10.4, or does Apple just really like the number 10? Did Linux stop getting major changes during the 2.4 series and suddenly start again with the release of 3.0? Is the Solaris 11 release more significant than the SunOS 5.11 release even though they are the same thing? The "real" meaning of a version number has always been whatever the developer wants it to mean.)