Mozilla themselves downplay the version numbers as meaningless. Yet, it's useful to be able to refer to a series of releases as all of a similar personality. Since 5 is 4, we should still be on a 4 series, until some kind of meaningful change to the personality has occurred.
I also wonder if they even tested it on slow connections. Firefox 5 has hung at least once a day (every about 5 hours of browsing time) and forced me to stop and reload the current page to finish downloading it. That was very, very rare on FF3.
I thought all of this had been said months ago when the new release schedule was announced. We all figured out what shorter time between major releases would mean: less would be added between major releases.
I feel like anyone who becomes outraged about version numbers is as shallow as the people Mozilla meant to appeal to in the first place. Would this post have been written at all if Firefox 5 had been Firefox 4.1? It's just a number.