Only a short time ago, we had to wait over a year for a major new release of any web browser. Often longer. There was a huge gap between Firefox 3 and Firefox 4, with many alphas, betas and numerous release candidates. That was before Google changed the game with Chrome, with rapid releases, quickly switching between stable, beta, dev and Canary.
Recently, Mozilla decided to change the way they would release a new version of Firefox, somewhat mimicking the Chrome release schedule. You can now download this stable version of Firefox, currently v9, while adventurous users can also check out v9 beta, v10 Aurora and even v11 of Nightly, Firefox's canary build, reserved for the most foolhardy.
There are also two minor improvements for OS X Lion users: improved theme integration and support for two-finger. swipe navigation. Otherwise it's a mix of stability fixes, better standards support and not much else. These incremental improvements build on the much bigger changes introduced in Firefox 8, including the following: downloads being integrated into the History window, an option for speeding up startup by only loading the currently selected tab when Firefox first loads (see the General tab of the program’s Options box).
Also introduced was tighter control of third-party add-ons. After you update to version 9, you'll be reminded of this when the compatibility assistant pops up for the second time since its introduction, allowing you to disable unwanted third-party add-ons the first time the program is updated to a new version. It also blocks third-party programs from installing add-ons silently.
Consequently, despite the disappointing lack of new features, Firefox 9 remains very much an essential update.
Note that this is the portable version of Firefox 9.