The company has baked in support for a number of new HTML 5 web standards, including offline storage support (ala Google Gears) as well as embedded audio and video. It also includes location aware browsing and an undo close feature can salvage not only lost tabs but closed windows as well. Firefox 3.5 beta 4 is expected to be the last beta and as such it primarily brings stability improvements, bug fixes and some more speed gains.
Additionally, Mozilla developers have announced a new stable version of the browser to address a critical security vulnerability introduced in Firefox 3.0.9 last week. The vulnerability is a regression which in some cases caused frequent crashes. More details about the release can be found in the release notes.