Setting xpinstall.signatures.required to false in the stable/release versions of Firefox doesn't do anything anymore. If you set it and try to install the .xpi file, it's still blocked from being installed. xpinstall.signatures.required only works with Nightly and Aurora (and maybe the Developer version) as far as I know, not the stable/release Firefox 49.0.1. Better question is why hasn't 2.4.14 been approved on AMO yet? Nonetheless, if you install the 2.4.14 beta on AMO, it *should* be the same version as the final.