Mozilla has had trouble making its development schedules. In 2008, the company originally shot for a late-2008 release of Firefox 3.5, but eventually to mid-2009 in order to add features and quash troublesome bugs in the then-new TraceMonkey JavaScript engine.

Among the new features in Firefox 3.6 are built-in support for the scaled-down browser skins dubbed "Personas;" warnings of ; support for new CSS, DOM and HTML 5 technologies; support for full-screen video embedded with the video HTML tag; and support for the Web Open Font Format (WOFF).

TraceMonkey has also been refreshed to boost JavaScript performance, something Mike Shaver, Mozilla's chief engineer, bragged about last week on Twitter. "I am excited about upcoming JS [JavaScript] engine work, and I don't care who knows it," .

As is usual for a Firefox preview, not all of the available browser's add-ons are compatible with the release candidate. According to Mozilla, about 75% can be used with Firefox 3.6.

Firefox controls about 25% of the global browser market, according to the most recent data from U.S.-based metrics company Net Applications. But while Firefox's usage share remained flat last month, surged into third place, pushing 's Safari into the No. 4 slot for the first time.