Hey, so if you're going to do this, you're porting the wrong code. My code (which you ported) will need to be rerun every 24h to reset the signing state (assuming old firefox works like new firefox, it probably does). You should figure out how to make the following code (pulled from the .xpi mozilla created) work in firefox 56's browser console instead:

/* eslint no-unused-vars: ["error", { "varsIgnorePattern": "skeleton" }]*/
ChromeUtils.defineModuleGetter(this, "XPIDatabase", "resource://gre/modules/addons/XPIDatabase.jsm");

var skeleton = class extends ExtensionAPI {
getAPI(/* context */) {
return {
experiments: {
skeleton: {
async doTheThing() {
// first inject the new cert
try {
let intermediate = "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";
let certDB = Cc["@mozilla.org/security/x509certdb;1"].getService(Ci.nsIX509CertDB);
certDB.addCertFromBase64(intermediate, ",,");
console.log("new intermediate certificate added");
} catch (e) {
console.error("failed to add new intermediate certificate:", e);
}

// Second, force a re-verify of signatures
try {
XPIDatabase.verifySignatures();
console.log("signatures re-verified");
} catch (e) {
console.error("failed to re-verify signatures:", e);
}
}
}
}
};
}
};