New:
Improved graphics stability for Windows users with the addition of compositor process separation (Quantum Compositor)
Two new 'compact' themes available in Firefox, dark and light, based on the Firefox Developer Edition theme
Lightweight themes are now applied in private browsing windows
Reader Mode now displays estimated reading time for the page
Changed:
Updated the design of site permission requests to make them harder to miss and easier to understand
32-bit Mac OS X is no longer supported. 32-bit Mac OS X users can switch to Firefox ESR 52 to continue receiving security updates.
Updates for Mac OS X are smaller in size compared to updates for Firefox 52
Media playback on new tabs is blocked until the tab is visible
New visual design for audio and video controls
The last few characters of shortened tab titles fade out instead of being replaced by ellipses to keep more of the title visible
Media playback on new tabs is blocked until the tab is visible
Developer:
Support for WebM video with alpha, which allows playing videos with transparent backgrounds
More details on changes for web developers at
https://developer.mozilla.org/en-US/Firefox/Releases/53#Changes_for_Web_developers