Firefox 66.0 will be released on March 19, 2019 to the Stable channel. The new Firefox version introduces new features such as sound autoplay blocking, a new storage format for extensions to improve performance, or support for the AV1 codec on Windows. For more info go here https://www.ghacks.net/2019/03/19/firefox-66-0-release-information/
Available through the builtin updater