If you’re wondering what happened to beta versions 1 through 3, they were labeled as Firefox 3.1, but Mozilla decided that the jump in features from 3.0.x were significant enough to make the numbers a bit further apart.
Although the latest versions of Chrome and Safari feature private browsing modes, Firefox 3.5 brings the most fully-featured implementation of it thus far.
The private browsing mode, enabled under the Tools menu, will not store any trace of pages visited, form or search entries, passwords, download lists (though downloads stored on the hard drive will remain), cookies and cache.
Clearly the first use for this mode is for those to feel less paranoid when browsing <ahem> more adult-oriented sites, but it can also be used for secretly shopping for gifts on shared computers.
Firefox 3.5 also adds geo-location support. The completely optional feature, which Mozilla promises is designed with the utmost care for the user’s privacy, uses IP addresses, wireless access points and GPS data (sent over SSL) to provide web services with location-specific information. A search for pizza will show results closest to you first, and mapping software will determine your starting point automatically.
The newest Firefox also incorporates support for new web technologies such as HTML5 <video> and <audio> elements, downloadable fonts and other new CSS properties, JavaScript query selectors, HTML5 offline data storage for applications, and SVG transforms.
Even if the new features don’t interest you, general browsing speed is improved with the new TraceMonkey JavaScript engine and improved Gecko layout engine, which includes speculative parsing for faster content rendering.
Download Firefox 3.5 Beta 4 here, though as with all beta software we recommend it only for experienced and adventurous users.