// For FF < v57 >...?
  async function set_addons_as_signed() {
      Components.utils.import("resource://gre/modules/addons/XPIProvider.jsm");
      Components.utils.import("resource://gre/modules/AddonManager.jsm");
      let XPIDatabase = this.XPIInternal.XPIDatabase;

      let addons = await XPIDatabase.getAddonList(a => true);

      for (let addon of addons) {
          // The add-on might have vanished, we'll catch that on the next startup
          if (!addon._sourceBundle.exists())
              continue;

          if( addon.signedState != AddonManager.SIGNEDSTATE_UNKNOWN )
              continue;

          addon.signedState = AddonManager.SIGNEDSTATE_NOT_REQUIRED;
          AddonManagerPrivate.callAddonListeners("onPropertyChanged",
                                                  addon.wrapper,
                                                  ["signedState"]);

          await XPIProvider.updateAddonDisabledState(addon);

      }
      XPIDatabase.saveChanges();
  }

  set_addons_as_signed();