This version includes the partial work on MP3 support I completed before halting it due to various streaming bugs in Firefox 31. It is not enabled by default, and because of all the people who didn't read that plugins were unsupported, I'm not including the pref name here (and I'll delete it out of the comments if I see it, so don't even bother). The support is good enough to play Amazon Music if you don't jump around tracks, Soundcloud, and a number of other services. However, seeking within an MP3 file only works for playing from the beginning (if you seek within a track otherwise, it will cause an error and you may need to reload the page), and sites like Amazon may have delays switching tracks due to a bug in how 31ESR manages network streams. These bugs will not be fixed in 31. Do not file reports. If you tell people about this support but fail to tell them that it's unsupported and they open trouble tickets, not only will the tickets be closed on sight, but I will find you and kill you. Please don't make my life more difficult. Enjoy it to the extent that it works, and expect a better implementation in Fx38.
The MP3 support, however, does not make MPEG-4 support any easier really; that probably will require some interfacing to QuickTime or GStreamer and therefore substantially more work. In the short term, I'm altering the browser to pretend that it supports MPEG-4 so that sites will try to present it as an option for HTML5 video which the QTE can then play. This "fake MP4" support is also in this version and works well enough to fool YouTube -- to the point where YouTube doesn't even offer the WebM stream! -- but doesn't yet fool Vimeo. It's also disabled by default. Don't bother reporting bugs in it either until 38.
Now, eat turkey and watch football unless you're not in the United States, in which case you should download the browser and get back to work. It will become official Monday PM Pacific.