Like IE9, Firefox now includes a feature to prevent sites from being able to track what you do online. This is a welcome addition, but it isn't without its problems. For one, it's an all-or-nothing feature--it's either on or off; there's no way to let some sites through and block others. And it isn't really universal yet: When you enable tracking protection in Firefox, it'll use what are called to tell the site that you don't want to be tracked. A simplified way of putting it is that when you visit a Website, Firefox will send a message to the site that you don't want to be tracked.

The problem is that , thus rendering the tracking protection feature useless. Mozilla is working to make this feature an industry standard, so hopefully things will improve in time.

Firefox 4 also sports a number of new features designed to improve page loading and rendering performance. Firefox 4 can take better advantage of your graphics card than Firefox 3.x could; using it to play videos, for example. Firefox 4 also includes an updated JavaScript engine that should improve performance of Web apps and certain elements of Web pages.

We haven't done any in-depth speed testing of Firefox 4, but we did test its JavaScript performance using the SunSpider 0.91 benchmark. Firefox 3.6 completed the test in 1103 milliseconds (ms) on average, which was the slowest among the major browsers (Safari, IE9, Chrome, Firefox, Opera). Firefox 4 completes the test in 363 ms on average, making it competitive with other current browsers (In my testing, all browsers averaged between 329 and 439 ms.)