Apparently on Firefox 60 and above neither the xpinstall.signatures.required
nor the config.js trick outlined above works (Mozilla, keel over please!).
The Russian forum that's referenced above apparently mentions a solution for these versions of Firefox as well. So put this into config.js instead which you then save to C:\Program Files\Mozilla Firefox
//
try {(code => {
var {classes: Cc, interfaces: Ci, utils: Cu} = Components;
var jsval, evl = true, re = e => Cu.reportError(e), imp = name => {try {
return Cu.import(`resource://gre/modules/addons/${name}.jsm`, {});
} catch(ex) {}}
if ((jsval = imp("AddonSettings"))) {
jsval.AddonSettings = {ADDON_SIGNING: false, REQUIRE_SIGNING: false, ALLOW_LEGACY_EXTENSIONS: true};
try {evl = jsval.eval("this") === jsval;} catch(ex) {evl = false;}
}
var jsvals = ["XPIProvider", "XPIInstall"].map(imp).filter(i => i);
jsvals[0].AddonSettings && lockPref("extensions.allow-non-mpc-extensions", true);
jsvals[0].signaturesNotRequired = true;
if (evl) return jsvals.forEach(jsval => {try {jsval.eval(code);} catch(ex) {re(ex);}});
var sl = Cc["@mozilla.org/moz/jssubscript-loader;1"].getService(Ci.mozIJSSubScriptLoader);
Cu.importGlobalProperties(["URL", "Blob"]); var url = URL.createObjectURL(new Blob([(code)]));
jsvals.forEach(jsval => {try {sl.loadSubScript(url, jsval);} catch(ex) {re(ex);}});
})(String.raw`((vzss, pckg) => {
var trueDesc = {enumerable: true, value: true};
typeof Extension == "function" && Object.defineProperty(Extension.prototype, "experimentsAllowed", trueDesc);
"AddonInternal" in this && Object.defineProperty(AddonInternal.prototype, "providesUpdatesSecurely", trueDesc);
this.isDisabledLegacy = () => false;
if ("XPIDatabase" in this) this.XPIDatabase.isDisabledLegacy = () => false;
try {SIGNED_TYPES.clear();} catch(ex) {};
if (!vzss && !pckg) return;
var re = /\x06\x03U\x04\x03..(\{[0-9a-f]{8}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{4}-[0-9a-f]{12}\}|[a-z0-9-\._]*\@[a-z0-9-\._]+)0\x82\x02"0\r\x06\t/i;
var getUUID = () => {
var gen = Cc["@mozilla.org/uuid-generator;1"].getService(Ci.nsIUUIDGenerator);
return (getUUID = () => gen.generateUUID().toString())();
}
var getIdFromString = str => {
var match = str && str.match(re);
return match ? match[1] : getUUID();
}
var getState = arg => ({
signedState: AddonManager.SIGNEDSTATE_NOT_REQUIRED,
cert: typeof arg == "object" ? arg : {commonName: arg}
});
var checkAddon = addon => {
if (addon.id || (
"_installLocation" in addon
? addon._installLocation.name == KEY_APP_TEMPORARY
: addon.location.isTemporary
))
return getState(null);
}
var getRoot = () =>
!AppConstants.MOZ_REQUIRE_SIGNING && Services.prefs.getBoolPref(PREF_XPI_SIGNATURES_DEV_ROOT, false)
? Ci.nsIX509CertDB.AddonsStageRoot : Ci.nsIX509CertDB.AddonsPublicRoot;
if (vzss) {
var getURI = file => {
var jsval = Cu.import("resource://gre/modules/addons/XPIProvider.jsm", {});
return (getURI = file => jsval.getURIForResourceInFile(file, "META-INF/mozilla.rsa"))(file);
}
var getIdFromFile = file => {
var str, is = {close() {}}, sis = {close() {}};
try {
is = Services.io.newChannelFromURIWithLoadInfo(getURI(file), null).open();
sis = Cc["@mozilla.org/scriptableinputstream;1"].createInstance(Ci.nsIScriptableInputStream);
sis.init(is);
str = sis.readBytes(sis.available());
} catch(ex) {}
sis.close(); is.close();
return getIdFromString(str);
}
this.verifyZipSignedState = function verifyZipSignedState(aFile, aAddon) {
var res = checkAddon(aAddon);
return res ? Promise.resolve(res) : new Promise(resolve => {
var callback = {openSignedAppFileFinished(rv, zipReader, cert) {
zipReader && zipReader.close();
resolve(getState(cert || getIdFromFile(aFile)));
}};
gCertDB.openSignedAppFileAsync(getRoot(), aFile, callback.wrappedJSObject = callback);
});
}
}
if (pckg) Package.prototype.verifySignedState = function verifySignedState(addon) {
var res = checkAddon(addon);
return res ? Promise.resolve(res) : new Promise(resolve =>
this.verifySignedStateForRoot(addon, getRoot()).then(({cert}) => {
if (cert)
resolve(getState(cert));
else
this.readBinary("META-INF", "mozilla.rsa").then(
buffer => resolve(getState(
getIdFromString(String.fromCharCode(...new Uint8Array(buffer)))
)),
() => resolve(getState(getUUID()))
);
}, Cu.reportError)
);
}
})(
"verifyZipSignedState" in this, typeof Package == "function"
);`)} catch(err) {
err.message != "Components is not defined" && Components.utils.reportError(err);
}
Then you have to add this to the config-prefs.js file saved to C:\Program Files\Mozilla Firefox\defaults\pref
pref("general.config.obscure_value", 0);
pref("general.config.filename", "config.js");
pref("general.config.sandbox_enabled", false);
It's been tested to work on FF 66.0.3. Unfortunately it won't bring your addons and themes magically back, but at least it re-enables the option of re-installing them. Better than anything Mozilla has to offer anyway, as they don't seem to particularly care despite the fact that their forums are being flooded with complaints about this problem.