Firefox 4 Beta 1 for Windows, Mac OS X and Linux is available to download. This version even if still under beta testing is mostly stable enough for regular use. The biggest improvements between Firefox 3 the newest one is speed.
Firefox 4 supports also CSS 3 and HTML5, it has also better protection against buggy plugins which crash the browser. The company plans to release a new beta version of the browser every two weeks. According to the newest data about Browser Market Share Firefox is the second (after IE) most popular browser on the market.