Firefox listed as incompatible (outdated APK | arm64-v8a)
The current link from which the Firefox and Firefox Beta APKs are downloaded from gives an outdated APK.
The link:
https://download.mozilla.org/?product=fennec-latest&os=android{arch}&lang=multi
It gives the 68.11.0 version which is out of support in 2020. The current version is 80.0.
Now my phone has an arm64-v8a architecture which is not supported by the 68.11.0 version (according to the table), thus the APK is incompatible.
A solution would be to use the CI site of Firefox: https://firefox-ci-tc.services.mozilla.com/tasks/index/mobile.v2.fenix.release.latest/ from which one can download the latest builds for each architecture. This solution is used also for the FFUpdater