Firefox 21 Aurora comes with several big new features, but there are plenty of improvements under the hood, of interest to developers.
WebRTC may be getting a lot of attention and it's clearly the big feature in Firefox 21, but its real-world applications are close to zero today.
Other, more minor updates and changes will have a bigger immediate impact though. Firefox 21 Aurora made some changes to the way the CSS -moz-user-select:none property is implemented, to make it more compatible with the WebKit implementation.
Actually, the change should bring Firefox in line with WebKit, IE and Opera, in this respect. It's probably not another sign of the WebKit monoculture taking over, but it's not hard to read it that way given the recent events.
The latest Aurora release adds support for the HTML5 <main> element which does pretty much what its name suggests, it defines the "main" area of a page, where the actual content goes, as opposed to the header or the footer sections.
Also new is support for scoped stylesheets which enable developers to more finely define the area which should be covered by a particular style.
Add-on libraries included in Firefox
Firefox 21 Aurora is also the first to incorporate the Add-on SDK loader and API libraries, meaning that extension developers won't have to include them in their creations which further means extensions will never rely on outdated API libraries, since they'll always be using the latest.
This is a fairly significant change and it could be disruptive for add-on developers. The benefit, once it's all over, is that developers won't have to repackage their creations for each new Add-on SDK version, just to include the latest API libraries.
This should mean less work for developers and should also mean extensions stay supported and work in Firefox for longer without any intervention or maintenance.