By pretty much any measure, Mozilla’s Firefox browser has been a huge success. Firefox is one of the most successful open-source applications of all time, second only to the Apache Web server. And in just a few short years, Firefox has been able to take significant market share away from Microsoft’s Internet Explorer, a task that seemed impossible when Firefox first launched.

But in the last year, Firefox has faced challengers in areas in which it was always comfortably ahead of IE, such as innovative new features, standards support and reliability. In many ways, the only claim to superiority that Firefox most recently had over rivals such as Apple Safari, Google Chrome and Opera was in its large community of add-ons.

However, with the release of Firefox 3.5, Mozilla has addressed many of the biggest problems of its Web browser. And, while most of the new features are under the hood, Firefox 3.5’s improvements are enough to push it back among the top Web browsers in all areas and to make it one of the more significant new browser releases. The improvements are also enough to gain Firefox 3.5 an eWEEK Labs Analyst’s Choice.

For images of Firefox 3.5 in action, click here. 

Among the biggest criticisms leveled at recent versions of Firefox have been slow performance and poor reliability, with many claiming that Firefox drags after long browsing sessions and that it is prone to crashing. To be fair, these problems were often due more to the add-ons used than to the browser itself, but, with Version 3.5, Firefox appears to have fixed most of these issues.

In my tests of the betas, release candidates and final version of Firefox 3.5, I have found the browser to be very stable. I’ve seen no noticeable slowdowns, even with large numbers of open windows and tabs.

And when it comes to performance, Firefox and its new browser engine look to have improved significantly. In multiple tests using online resources, including Futuremark’s Peacekeeper benchmark, Firefox 3.5 showed considerable performance gains, more than doubling the speed of Firefox 3.0. And while it still lags behind performance leaders such as Safari 4 and Chrome 2.0, Firefox 3.5 is now much more comparable.