Maighstir: That jump came solely because Firefox decided to jump on Chrome's version train of "don't bother with point-releases, make each release a new full version". So after 3.6.18, they jumpeed to 4, to 5, to 6... to 29 now (Chrome is up to 30-something).
Yes, I recall the joke about jealousy being the reason for the updates, though it still is ridiculous. Using FF 3.6, updating to 4.x, have a problem with a website, then being advised to update to FF 6.x, while you wonder when the hell was version 5.x released, and when was it made obsolete.