Firefox Beta and Developer Edition
Release Notes

Pinned tiles on the new tab page can be synced

Support for the full HTTP/2 protocol. HTTP/2 enables a faster, more scalable, and more responsive web.

New preferences implementation enabled for the first half of the beta cycle

Locale added: Uzbek (uz)

CSS gradients work on premultiplied colors

Fix some unexpected logout from Facebook or Google after restart

The Flash protected-mode sandbox is disabled on Windows, to evaluate the stability impact of protected mode

-remote option removed

No longer accept insecure RC4 ciphers whenever possible

Phasing out Certificates with 1024-bit RSA Keys

Shut down hangs will now show the crash reporter before exiting the program

Add-on Compatibility

Eval sources now appear in the Debugger
Debug JavaScript code that is evaluated dynamically, either as a string passed to eval() or as a string passed to the Function constructor

DOM Promises inspection

Inspector: More paste options in markup view

Implemented a subset of the Media Source Extensions (MSE) API to allow native HTML5 playback on YouTube (Windows Vista or later only)

Support for the ECMAScript 6 Symbol data type added

unicode-range CSS descriptor implemented

CSSOM-View scroll behavior implemented allowing smooth scrolling of content without custom libraries

object-fit and object-position implemented.
Defines how and where the content of a replaced element is displayed

isolation CSS property implemented.
Create a new stacking context to isolate groups of boxes to control which blend together

CSS3 will-change property implemented.
Hints the browser of elements that will be modified. The browser will
perform some performance optimization for these

Improved ES6 generators for better performance

Changed JavaScript 'const' semantics to conform better to the ES6 specification.
The const declaration is now block-scoped and requires an initializer. It also can not be redeclared anymore.

<meta name="referrer"> implemented for more privacy. Allows to control the content of the HTTP referer header.