There was also supposed to be a new feature for delaying video decoding until the rest of the page had loaded to improve throughput on YouTube and some other sites, but YouTube introduced its new site design while I was testing the feature, and unfortunately the "lazy loading" technique they appear to be using now means the browser cannot deterministically compute when video will start competing with layout for resources. I'm thinking of a way to retool this but it will not be an enabled part of FPR3. One idea is to forge dropped frames into MSE's statistics early so it shifts to a lower quality stream for a period of time as a "fast start;" another might be to decouple the media state machine from the decoder more completely. I haven't decided how I will attack this problem yet.
In miscellaneous changes, even after desperate begging Imgur would not fix their site sniffer to stop giving us a mobile version using the default TenFourFox user agent (this never used to happen until recently, btw), even if just an image by itself were requested. I got sick of being strung along by their tech support tickets, so this version just doesn't send any user agent to any Imgur site, unless you explicitly select something other than the default. Take that, Imgur. The reason I decided to do this for Imgur specifically is because their mobile site actually causes bugs in TenFourFox due to a hard CoreGraphics limit I can't seem to get around, so serving us the mobile site inappropriately is actually detrimental as opposed to merely annoying. Other miscellaneous changes include some widget tune-ups, more removal of 10.7+ specific code, and responsiveness tweaks to the context menu and awesome bar.
Last but not least, this release has a speculative fix for long-running issue 72 where 10.5 systems could end up with a frozen top menu bar after cycling repeatedly through pop-up menus. You'll notice this does not appear in the source commits yet because I intend to back it out immediately if it doesn't fix the problem (it has a small performance impact even on 10.4 where this issue does not occur). If you are affected by this issue and the optimized build doesn't fix your problem, please report logging to the Github issue from the debug version when the issue triggers. If it does fix it, however, I will commit the patch to the public repo and it will become a part of the widget library.
Other than that, look for the final release on or about September 26. Post questions, concerns and feedback in the comments.