Type 'about:config' in the location bar.
extensions.xnotifier.saveCookies
When it is set to 'true', X-notifier's cookies are saved
in Firefox's cookie manager. (default)
When it is set to 'false', X-notifier's cookies are not saved.
They are on memories, while Firefox is running.
After you set (true or false), please restart Firefox.
Tobwithu explain more clearly for what purpose
by saving cookie on Firefox.