An OS owns and can enforce policies around the filesystem. For example, a future release of macOS might say that files cannot be both executable and writable, a

An OS owns and can enforce policies around the filesystem. For example, a future release of macOS might say that files cannot be both executable and writable, and the only way a file can be marked executable is by the security system validating a signature, or an administrative user override. Firefox has to deal with malware injecting extensions outside the normal browser process, so I'm not surprised they would default to having periodic re-checks.