14 DAY TRIAL // Firefox 15 landed in the beta channel along with some new features and plenty of improvements. Firefox 15 debuts the PDF viewer Mozilla's been working on for a year now. The tool is enabled by default, but it's still labeled as experimental.
Also new is support for SPDY/3, the latest draft version of the updated protocol and a huge improvement in memory usage when add-ons are involved.
One of the biggest improvements is a fix for memory leaks caused by add-ons. The fix contains the biggest leaks in add-ons preventing them from using up more and more memory in time.
The nice part is that this works even if the add-ons are faulty, third-party developers don't have to do anything to benefit from this. That's not to say that they shouldn't care about leaks and memory usage from now on. You can read a more detailed explanation of the improvements here.
The special thing about the PDF viewer is that it's built entirely in JavaScript meaning that all the content is rendered by the browser just like any other page.
The project is open source too, so anyone can implement it in his or her own apps. There's still some work to do be done and you may still need a third-party reader for more complex files.
Firefox 15 also adds support for the latest draft spec of the SPDY protocol, which is on its way to becoming HTTP 2.0, judging by all the support it gained so far. SPDY/3 is already used by Google servers and others will be deploying it as well.
The latest Chrome releases also support SPDY/3. Firefox has put its weight behind SPDY as a candidate for HTTP 2.0 and has said it will start deploying it with its servers soon, albeit the older SPDY/2 version.