Firefox 3.5 brings the world's second-most popular browser up to speed with current browsing technology and trends, and perhaps nudges it just a bit ahead of the competition. However, it is by no means the leap ahead that its predecessor Firefox 3 was, and it's clear that the competition isn't going away anytime soon.

Available for Windows, Windows Portable, Mac, or Linux, Firefox 3.5 nevertheless represents the best Firefox we've yet seen from Mozilla. This comes as no surprise, and with a testing process that involved four beta builds, three release candidates, and a version change to reflect what Mozilla described as the originally-unintended breadth of the improvements being made, most of the new features are no surprise, either.

Private Browsing, known to IE users as InPrivate, Chrome users as Incognito, and Safari users as, well, Private Browsing, finally comes to a public version of Firefox. It's been available to the 800,000 or so beta testers since December 2008. If you're not familiar with it, users can toggle on or off the browser's history, cookies, and other browsing traces at will via the Tools menu or CTRL+SHFT+P. A new window will open. Among its other uses that serve as fodder for second-rate comedians, it's an excellent tool for avoiding leaving tracks on publicly-used computers and its about time that Firefox finally got it. In fact, Firefox has had it in various stages of development for four years.

I'm not sure how connected Firefox's development of Private Browsing is to this next feature, but I can see far more users gaining traction from having the fine, granular control of browsing tracks that's now available in v3.5. The Clear Private Data window has been replaced by a Clear Recent History option, using the same hot key combo and in the same place in the Tools menu.

Under the Clear Recent History window, you can delete your entire recent browsing history over the past hour, two hours, four hours, today, or all content in your history. From its Details drop-down menu, you can tailor the data purge to Browsing and Download history, Form and Search history, Cookies, Cache, Active Logins, Site Preferences, and Saved Sessions. From within the History window, you can also right-click on a site to Forget this Site, which will remove all instances of that site from your history records. Because your Most Recent Sites folder pulls from your history, you gain this level of control there, too.

Another excellent improvement in v3.5 that pushes Firefox ahead of its competitors is aggressive developer support. This may not sound impressive to most users, and if you're not a developer, I can see why its hard to get worked up about support for CSS media tags, HTML5 local storage, downloadable fonts, Web worker thread, and native JSON support, or SVG transforms--it all sounds a bit too much like alphabet soup.

Firefox 3.5 comes with geo-locating turned on, so it always knows where you are (with your permission.) (Credit: Screenshot by Seth Rosenblatt/CNET)

However, embedded ICC profiles, and support for Ogg Vorbis and Theora video and audio means that image colors will look better and closer to how they were intended, and no plug-in will be required for properly-encoded multimedia. Since Vorbis is open-source, this will lend those formats a huge boost while rendering those pages more stable. Here's an example video from Firefox that offers a tour of the new browser, or you can check out this sample from Daily Motion. Non-Firefox users will either see the Flash version (as on Daily Motion), or be directed to download the OGV file.

The "awesome bar" that debuted in Firefox 3 has become one of my favorite features. I've personalized my browser to eliminate the search bar, and now I use the location bar for all my searching. In v3.5, Mozilla has improved the search functionality so that you can show only bookmarks, by using an asterisk after a query such as "cnet *", or show only tags by using a plus "cnet +".

You can also tear off tabs as you can in the Webkit-based browsers Chrome, Safari, and IE, although unlike those browsers, Firefox's tabs are not sandboxed. This means that, if the browser crashes, you're still hosed, although Mozilla says this feature--known in development as Electrolysis--is being worked on.

In the meantime, Mozilla has imported better session control that users could only get before from add-ons like Session Manager. Now, if Firefox crashes, you get the option to choose which tabs to revive. If a Flash-based or heavy JavaScript site was the cause of that crash, you don't need to bring back that particular tab and risk getting caught in a crash-and-restart cycle of frustration.

Firefox 3.5 natively supports HTML5 and embedded Ogg video content. (Credit: Screenshot by Seth Rosenblatt/CNET)

Mozilla abandoned development of its own geolocating technology in Firefox, but that doesn't mean that Firefox 3.5 doesn't possess the ability to know where you are. Using Google's tech, Firefox can pinpoint where you are so that in search queries, for example, you'll get the most locally relevant results first. Turning this off isn't difficult, either. Under about:config, search for "geo.enabled" and change True to False by double-clicking on it.

Performance has always been one of the keys to browser popularity, and much of Google's success with Chrome can be attributed to its fast JavaScript rendering marks. The resurgent interest in Safari also comes from its JavaScript benchmarks and Apple's claim that Safari is the fastest browser on the market with its Nitro JavaScript engine. Firefox 3.5 doesn't beat them on the JavaScript front, but it's within shooting range.

On a Lenovo T400 laptop with a Core 2 Duo T9400 processor running at 2.53 GHz, with 3 GB of RAM and Windows 7 RC 7100, I ran the SunSpider JavaScript test and Dromaeo's subset of JavaScript tests on Firefox 3.0.11, Firefox 3.5, Internet Explorer 8, Chrome 2, and Safari 4. As much as I like Opera as an all-in-one browser, I left it out because Opera 9.6 hasn't stood up well to the improvements that the field has made in the past year, and Opera 10 beta isn't ready to be compared to public releases at this point. Remember that for SunSpider the lower number is better, while the opposite is true of Dromaeo.

Firefox users can now rip tabs off into new windows, or drag them back into the old one. Still no sandboxing, though. (Credit: Screenshot by Seth Rosenblatt/CNET)

Firefox 3.0.11 completed SunSpider in 2695.4 milliseconds, and 44.22 runs per second, while Firefox 3.5 notched 1319.6 ms on SunSpider and 91.18 runs/s. This falls in line with Mozilla's published benchmarks of 3669 ms for Firefox 3 versus 1524 ms for Firefox 3.5. In both "official" numbers and in my own tests, Firefox 3.5 comes out around twice as fast for JavaScript.

Meanwhile, Chrome 2 hit 322.1 runs/s on Dromaeo and 712.2 ms on SunSpider. Either way, Chrome is significantly faster than Firefox for JavaScript, one-third faster judging by SunSpider and twice as fast by Dromaeo. Safari 4 scored 915.6 on SunSpider and 239.02 runs/s on Dromaeo, slightly slower than the its Webkit cousin Chrome but still faster than Firefox. Internet Explorer marked 4434.6 ms in SunSpider, but crashed on Dromaeo while testing base 64 encoding and decoding.

Firefox 3.5 is around twice as fast as Firefox 3. Chrome and Safari are faster with JavaScript, though. (Credit: Screenshot by Seth Rosenblatt/CNET)

It's important to note that speed is not the only criterion for judging a decent browser. Each browser only had open two tabs, the results of its Dromaeo test and the results of its Safari test. Safari consumed nearly 135 MB of RAM, IE saw 104 MB, Firefox 3.5 hit 66 MB, and Chrome logged 46.5 MB. These results will fluctuate depending on your computer and any other tasks your browser is running at the time, but they give a decent idea of how each browser is performing during these tests.

Other useful tests look at Web standards rendering, like the Acid3, and deeper analysis of the SunSpider results. Chrome and Safari both reach 100/100 on the Acid3 test, while Firefox makes it to 93/100. Official release notes for Firefox 3.5 can be read here.

Firefox 3.5 is a much-needed improvement to the world's most popular alternative browser. At the time of writing, Mozilla was about to log the 2 millionth download after only 7 1/2 hours. While some of the improvements, such as the HTML5 and other developer enhancements will continue to make the browser their first choice, many of the other changes merely keep it in-line with the competition. For now, Firefox will continue to rely on its vast base of developers and users who value their customizations over superlative claims, so long as Mozilla keeps its browser close enough to its competitors. Now that Firefox has kicked open the door against Internet Explorer, it'd be foolish to expect that they'd be the only ones to rush through it.