Version 63.0beta, first offered to Beta channel users on September 5, 2018
Added content blocking, a collection of Firefox settings that offer users greater control over technology that can track them around the web. In 63, users can opt to block third-party tracking cookies or block all trackers and create exceptions for trusted sites that don’t work correctly with content blocking enabled.
WebExtensions now run in their own process on Linux
Firefox now warns about having multiple windows and tabs open when quitting from the main menu. The Save and Quit feature has been removed. You can restore your session by ticking the box for Restore previous session in the General → Startup options or by using Restore Previous Session in the main menu.
On macOS, WebGL power preferences allow non-performance-critical applications and applets to request the low-power GPU instead of the high-power GPU in multi-GPU systems
Improved Windows 10 integration: your Firefox theme now matches your OS Dark / Light mode
Faster tab switching in the majority of cases for our macOS users
Improved the reactivity of Firefox on macOS
Resolved an issue that prevented the address bar from autofilling bookmarked URLs in certain cases
In the Library, the Open in Sidebar feature for individual bookmarks was removed
The option to Never check for updates was removed from about:preferences. You can use the DisableAppUpdate enterprise policy as a substitute.
The Ctrl+Tab shortcut now displays thumbnail previews of your tabs and cycles through tabs in recently used order. This new default behavior is activated only in new profiles and can be changed in preferences.
The build infrastructure of Firefox on Windows moved to the Clang tool chain, bringing important performance gains
Refreshed visual style of Developer Tools menus to improve navigation and consistency
The Dev Tools accessibility inspector is now enabled by default. This tool surfaces information exposed to assistive technologies on the current page, allowing you to check what’s missing or otherwise needs attention.