Perhaps most notable for Linux Firefox users is support for VP9 video decoding has been integrated. Besides having VP9 video decoding, Firefox 28 also has support for Opus audio within WebM. Further improving the open video support in Firefox, this new release has volume controls for HTML5 audio.video elements. As a bit for helping browser gamers, the Firefox Gamepad API has also been finalized and enabled.
Other Firefox 28 changes include the removal of SPDY/2 support, support for MathML 2.0's mathvariant attribute, background thread hang reporting, an input type of color, and support for multi-line flexboxes in the layout.
The Mozilla Firefox 28.0 announcement has yet to be issued but the Linux x86_64 binary can be fetched right now via this Mozilla.org link.