Add support for SeaMonkey 2.48+ (including current nightly) and Firefox 51+ (including current nightly).
Depending on which nightly you are using, you may need none or all of these flags in your user.js file:
user_pref("extensions.allow-non-mpc-extensions", true);
user_pref("extensions.legacy.enabled", true);
user_pref("xpinstall.signatures.required", false);
user_pref("extensions.checkCompatibility.nightly", false);