For starters, it's faster thanks to the new Tracemonkey JavaScript engine. As I've written before, TraceMonkey in my view is a critically important feature, perhaps the most important new feature in Firefox 3.5 as a whole. The general idea is that it is faster than the JavaScript engine in Firefox 3.x and is competitive with Apple Safari's  Nitro JavaScript and Google Chrome's V8 JavaScript engines. Faster JavaScript engines means faster code execution means faster web experiences.

Mozilla has also baked in support for a number of new HTML 5 web standards, including offline storage support (think Google Gears) as well as embedded audio and video.

Private Mode browsing (AKA Porn mode) is also part of Firefox 3.5, bringing that same feature that Google Chrome, Apple Safari and IE 8 users already have to mainstream Firefox users.

From a web developer point of view, the list of improvements to Firefox 3.5 is a long one. Basically what it amounts to are new CSS, Ajax (XHR and JavaScript), DOM and HTML 5 features. Yeaah I know it's a lot.

I personally would like to see an comparison grid (or maybe I should make one) of how the major browsers now all support CSS -- since simply saying CSS support (3.x etc) doesn't mean much. From a development point of view, it's right down to the individual functions now. That's not a good thing in my view. It is too hard for most mainstream developers to use a feature that is only supported by one browser, and not all. What ends up happening is the 'lowest common denominator' rule, where devs will only use the features that are common amongst all browsers.

Then again, by advancing both CSS and HTML 5 features, eventually it does tend to advance the bar for all.

As far as when I'd speculate Firefox 3.5 will be ready for public consumption, I'd venture to guess at this point we're still a few months out. This Beta 4 release follows Beta 3 by about six weeks, there easily could be a Beta 5 and then there are the Release Candidates yet to come.