I tried cleaning my history, removing the script and re-adding it and even tried a brand new profile on firefox. It just continues to say Google+ not checked.
Not that this helps much, but... (and I don't get many messages on Google+, but) my account in Firefox X-Notifier does not report a problem checking for new messages.
Note
Not that this helps much, but... (and I don't get many messages on Google+, but) my account in Firefox X-Notifier does not report a problem checking for new messages.
I would have thought a new profile would have worked, but...
Have you done everything listed in http://xnotifier.tobwithu.com/dp/node/4