All extensions disabled due to expiration of intermediate signing cert

I have disabled signature checks in Firefox because otherwise it is impossible to install a private extension without uploading the source code to Mozilla.

This is how you allow unsigned extensions in Firefox on Arch Linux, the same files can be edited on Windows and macOS, restart the browser after changes:

  sudo tee /usr/lib/firefox/defaults/pref/config-prefs.js &>/dev/null <<EOF
  pref("general.config.obscure_value", 0);
  pref("general.config.filename", "config.js");
  pref("general.config.sandbox_enabled", false);
  EOF

  sudo tee /usr/lib/firefox/config.js &>/dev/null <<EOF
  // keep this comment
  try {
    Components.utils
      .import('resource://gre/modules/addons/XPIDatabase.jsm', {})
      .XPIDatabase['SIGNED_TYPES'].clear();
  } catch (ex) {
    Components.utils.reportError(ex.message);
  }
  EOF

This method also works for the stable version of Firefox.