Support of Prefer:Safe http header for parental control.
mozilla::pkix as default certificate verifier (learn more)
Block malware from downloaded files (learn more)
Partial implementation of the OpenType MATH table (section 6.3.6) see documentation about mathematical fonts and the MathML Torture Test for details
audio/video .ogg and .pdf files handled by Firefox if no application specified (Windows only)
Upper Sorbian [hsb] locale added
Removal of the CAPS infrastructure for specifying site-specific permissions (via capability.policy.* preferences). Most notably, attempts to use this functionality to grant access to the clipboard will no longer work. The sole exception is the checkloaduri permission, which may still be used as before to allow sites to load file:// URIs.
WebVTT implemented and enabled (learn more)
CSS3 variables implemented (learn more)
Developer Tools: Add-on Debugger (learn more)
Developer Tools: Canvas Debugger (learn more)
New Array built-in: Array.prototype.fill() (learn more)
New Object built-in: Object.setPrototypeOf() (learn more)
CSP 1.1 nonce-source and hash-source enabled by default
Developer Tools: Eyedropper tool added to the color picker (learn more)