Some notes:
- Firefox 4 took a long time to come out, because it's powered by the first major revision of Gecko: 2.0. It entailed a MASSIVE rewrite. No that it's done, tacking or basing new features on it got much easier - thus the accelerated release schedule.
- When you install a new Firefox version, and said Firefox starts crashing for no reason, you should backup your bookmarks, security certificates etc. and wipe out your profile completely - extensions included. I know that on a computer I had, which started with Firefox 1.5, moving to 2.0 then 3.0 and then 3.5 and 3.6, sometime along the way I had to do this to make it stable. Once done however, it was rock solid.
- Firefox 4 uses hardware accelerated rendering: ensure that you install IE9 (it comes with a bunch of Direct2D patches) and all the latest Vista/7 patches, or (on WinXP and Vista) the latest DirectX version. On all platforms, get the latest release for your display driver.
- right now in the works are tab isolation (the first results of that branch were seen in 3.6.4, with plugin isolation), hardware acceleration on more platforms, better use of the cloud, and a furthering of the interface rework; these alone can justify a major revision.
- the previous release cycle used at Mozilla required one main developer branch (Minefield) whcih forked regularly for the stable releases; due to the monolithic nature of Gecko 1.x, this was the most practical solution; however, it also meant that developing a new feature required creating a fork, work on said fork, and rebase the work on Minefield regularly - which is time consuming, complex, prone to errors and release-unfriendly. The new system would allow better sync'ing between the different development branches, and go better with a rapid release cycle.