Firefox 42.0 releases today, a major version that brings with it some significant changes and feature additions.
The big one is Tracking Protection when Private Browsing. As Mozilla puts it, "We first added Private Browsing to Firefox to give you control over your privacy locally by not saving your browser history and cookies when you close a private window. However, when you browse the Web, you can unknowingly share information about yourself with third parties that are separate from the site you're actually visiting, even in Private Browsing mode on any browser. Until today." Note that Firefox already offered the "Do Not Track" feature for normal browsing -- it's unclear how this is different if at all from Tracking Protection.
Beyond that, you get a new Control Center which houses site security and privacy controls, one-click tab muting, Login Manager improvements to make it easier to login to websites, better performance on "interactive" websites, additional HTML5 support, and security fixes, to name most of it.