Quick question re: MyBB's cookies.

[Firefox Wins]

The same 'cheap and easy' SSO only works in the front-end and not for admin sessions. Instead of asking 'how can I make it work?'...  

What is the root cause of the difference?

Can anyone help provide a starting-point?
If front- end sessions are '(re-)authenticated' based on the login key, then what about admin sessions:

• Other than the session time-out (auto-logout) how is the cookie (session) verification different for the (admin) back-end?
  ...and/or...
• Why are 2 admin logins, using the same cookie path, causing a conflict, and logging the 'other one' out?
  

Thanks a bunch.

ps. Let me guess:
This one actually uses a unique session as part of the verification, is that correct?