Firefox 5 is a worthy expression of Mozilla's ideals. The browser is competitively fast, sports a new minimalist look, and includes some excellently executed features. Unfortunately, that describes most of Firefox's competition, too.
Firefox 5 is the first version of Firefox released under Mozilla's rapid-release cycle, which means that instead of seeing a new whole-number version once every year or two, you'll now see one every three months. This follows in the footsteps of Google Chrome, which adopted the rapid-release cycle a while ago and is now up to version 12 (at the time this review was written) despite having its first release only in 2008.
Firefox 4 had a massive impact on Firefox 5, and so this review is not dramatically different from its predecessor. Firefox 4 had a rough time in its early development, but those days are over. The browser that you can download now is in the same speed category as its competition; offers many similar features (stronger in many areas and slightly weaker in others); includes broad, cross-platform support for hardware acceleration and other "future-Web" tech and standards; and is a must-have for Android users (download for Android).
Also under the hood, CSS animations now work in Firefox 5, which means that the browser can handle more dynamic Web content that moves around a page more easily.
Not much of the interface has changed from Firefox 4, although the Do Not Track box has been moved to a more prominent place in the Options menu, under Privacy. It's also now available in Firefox for Android.