The release notes lists all changes implemented on this version:
Windows: Firefox now follows display scaling options to render text larger on high-res displays
WebRTC is now enabled by default!
Mac OS X: Download progress in Dock application icon
HTML5 audio/video playback rate can now be changed
Social services management implemented in Add-ons Manager
asm.js optimizations (OdinMonkey) enabled for major performance improvements
Improved WebGL rendering performance through asynchronous canvas updates
Plain text files displayed within Firefox will now word-wrap
For user security, the |Components| object is no longer accessible from web content
Improved memory usage and display time when rendering images
Pointer Lock API can now be used outside of fullscreen
CSS3 Flexbox implemented and enabled by default
New Web Notifications API implemented
Added clipboardData API for JavaScript access to a user's clipboard
New built-in font inspector
New HTML5 <data> and <time> elements
Scrolling using some high-resolution-scroll aware touchpads feels slow (829952)