One of the most notable changes in this release is an extra layer of security that will prevent plugin content from loading by default. From now on, all plug-ins operate on a "click to play" basis, only Flash is exempt from this rule.
NEW: All Java plug-ins are defaulted to 'click to play'
NEW: Password manager now supports script-generated password fields
NEW: Updates can now be performed by Windows users without write permissions to Firefox install directory (requires Mozilla Maintenance Service)
NEW: Support for H.264 on Linux if the appropriate gstreamer plug-ins are installed
CHANGED: Support for MP3 decoding on Windows XP, completing MP3 support across Windows OS versions
CHANGED: CSP implementation now supports multiple policies, including the case of both an enforced and Report-Only policy, per the spec
DEVELOPER: Social API now supports Social Bookmarking for multiple providers through its SocialMarks functionality (see MDN docs)
DEVELOPER: Math.ToFloat32 takes a JS value and converts it to a Float32, whenever possible
DEVELOPER: There is no longer a prompt when websites use appcache
DEVELOPER: Support for the CSS image orientation property
DEVELOPER: New App Manager allows you to deploy and debug HTML5 webapps on Firefox OS phones and the Firefox OS Simulator
DEVELOPER: IndexedDB can now be used as a "optimistic" storage area so it doesn't require any prompts and data is stored in a pool with LRU eviction policy, in short temporary storage
FIXED: When displaying a standalone image, Firefox matches the EXIF orientation information contained within the JPEG image (298619)
FIXED: Text Rendering Issues on Windows 7 with Platform Update KB2670838 (MSIE 10 Prerequisite) or on Windows 8.1 (812695)
FIXED: Improved page load times due to no longer decoding images that aren't visible (847223)