Firefox 51 isn't a big feature release for end-users but notably does have support for FLAC audio, at long last! Great to see the web browsers finally shipping support out-of-the-box for this open-source audio codec.
Also notable for Firefox 51 is WebGL 2.0 by default. Also, they have enabled Skia content rendering by default in the Linux build too.
So overall it's a pretty decent release if you get excited about the technical changes covered. Download Firefox 51 from Mozilla.org.