[jeroenhd]

On the other hand, Mozilla has fewer resources to spend on maintaining backwards compatibility. Mojave is pretty old as far as continuously updated operating systems are concerned, and the devices that didn't get the Mojave update are all over 10 years old at this point.

The hardware itself is supported just fine; install Linux on your 2011 Macbook and you can download the latest and greatest Firefox release. Mozilla just doesn't spend much time on operating systems that have fallen out of support. I don't expect them to keep their Windows 7 builds going for long once Microsoft ends their paid support programme.

If Apple doesn't bother publishing regular security and maintenance updates, then why should Mozilla? Users shouldn't be running operating systems that receive no or only some security patches anyway.

[shuffel]

Upgrading or switching the OS is a major undertaking.

I appreciate that Mozilla has fewer resources than Google, but having the latest version of FireFox working only on macOS versions released within the past two years is unnecessarily restrictive in my opinion. I'm an independent software developer far more financially constrained than Mozilla who supports macOS releases going back 10 years. It's just a matter of not using the new API calls shipped with each new OS release and optionally dynamically loading the API calls that are not present on all OS versions.

EDIT: My original post was wrong. Mozilla still supports Mojave. See details above.