The current beta version number of the Firefox release is tagged as 3.1. When Beta 4 is released next month it'll become version 3.5 and is tentatively set to be released sometime in June - but that could change depending on circumstances.

When it officially releases later this year it'll sport a new private browsing mode similar to those seen in IE8, Chrome, and Safari. Private browsing is a "globally toggle mode" (e.g. on/off) that is supposed to cover your tracks so that when you close the browser all traces of your activity are erased. The benefit is that someone else inspecting your system won't be able to easily recover tidbits that show what you were up to.

When in private browsing mode data such as cached pages, cookies, site visitation history, etc, and are all tracked. When erased details stored prior to enabling the mode remain in tact.

As you can see in the tracking ticket for this feature, development has progressed at a steady pace.