not exaclty an answer to you question … but just fyi you could also just create an AMO account (it’s free) and upload your addon for self distribution (to not have them appear on addons.mozilla.org) and then you can download and install them into stable version persistently.
sidenote: i think instead of nightly you can also use the unbranded builds to have a stable version which allows you to use xpinstall.signatures.required. ref. wiki.mozilla.org/Add-ons/Extension_Signing#Unbran…