You can go to the add-on's options page to remove a tab.
1. Menu bar -> Tools -> Add-Ons... (or Ctrl+Shift+A)
2. Find Morning Coffee Quantum in the list and click the Options button beside it.
3. Choose the day the site was added to.
4. Find the site in the list and highlight it.
5. Click Delete, then click Save.

If that helps, is there any chance I can have that fifth star back? ;)