Huh, never knew that. I wonder if that's because it's harder for Android apps to edit the settings of other Android apps (unless they have root access, but that's much more rare on Android than, say, Windows or Linux or macOS)?

Mozilla's official Linux builds disallow xpinstall.signature.required = false (last I checked), but the unbranded builds (as well as builds provided by at least some repos) do indeed allow signature bypassing.