With Firefox 115, users on macOS 10.12 (Sierra), macOS 10.13 (High Sierra) and macOS 10.14 (Mojave) will automatically be moved to the Firefox Extended Support Release (ESR).
While Apple doesn’t have an official policy governing security updates for older macOS releases, their recent practice has been to support the most recent three releases. The last security update for macOS 10.14 was nearly two years ago in July 2021. Moving users to the ESR 115 version of Firefox will allow Mozilla to continue providing security updates until at least September 2024. Users do not need to take additional action to receive these updates.
We strongly encourage users to upgrade to a version of macOS that is supported by Apple. When migrating to a new device, users can use a Firefox Account to easily keep all of their settings, ensuring all bookmarks, browsing history and saved passwords are immediately available after installing Firefox.
Unsupported operating systems receive no security updates and can be dangerous for you to use. For planning purposes, enterprises using Firefox should consider September 2024 as the support end date for macOS 10.12, 10.13 and 10.14.