Mozilla has issued Firefox 78, which is the last Firefox release that will support macOS 10.9 Mavericks, 10.10 Yosemite, and 10.11 El Capitan. If you’re using one of those operating systems, Mozilla will automatically move you to version 78.x of the Extended Support Release (ESR) edition for the coming year. Mozilla also encourages users to upgrade to macOS 10.12 Sierra or higher to take advantage of the security and privacy updates in its standard Firefox releases.
Firefox 78 also updates the Protections Dashboard with consolidated reports about tracking protection, data breaches, and password management; ensures screen savers no longer interrupt video calls using WebRTC; and incorporates various security fixes. (Free, 71.0 MB, release notes, macOS 10.12+)