Version 28 is an Rapid Release Cycle (RRC), for which Mozilla provides about 6 weeks support.
The new features include support for VP9 video decoding and the OS X notification center, so that notifications from web apps can now appear there. Also new is support for volume control for HTML5 audio and video and support for WebM Opus audio.
All of these updates are for both the desktop and Android version.
In addition, the mobile app is also getting support for native text selection, cut and copy, as well as additional quick share buttons and predictive lookup for Awesomebar entries.
Bug fixes and other changes can be found here: http://www.mozilla.org/en-US/firefox/28.0/releasenotes/
You can download FF 28 RRC from here: http://www.mozilla.org/en-US/firefox/all/
Mozilla Firefox 28 RRC has been successfully tested with NetWeaver versions 740,731, 730, 711,702, 701,700 on Win 7 64bit, Win 7 32bit, Win 8 64 bit(desktop), Win 8 32 bit(desktop), Win XP, Mac and Linux.
As usual, for further details and SAP restrictions see: