On June 30th, macOS 10.9, 10.10 and 10.11 users 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 ongoing practice has been to support the most recent three releases (i.e. version N, N-1, and N-2). The last security update applicable to macOS 10.11 was made available nearly 2 years ago in July 2018 (https://support.apple.com/en-us/HT201222). Unsupported operating systems receive no security updates, have known exploits, and can be dangerous to use, which makes it difficult and less than optimal to maintain Firefox for those versions.
Users do not need to take additional action to receive those updates. On June 30th, these macOS users will automatically be moved to the ESR channel through application update.
In the meantime, we strongly encourage our users to upgrade to mac OS X 10.12 or higher to benefit from the security and privacy updates.
For more information please visit the Firefox support page.