If clearing Cookies didn't help then It is possible that the Cookies.sqlite file in the Firefox profile folder that stores the Cookies got corrupted.

• rename/remove Cookies.sqlite (Cookies.sqlite.old) and if present delete Cookies.sqlite-shm and Cookies.sqlite-wal in the Firefox profile folder with Firefox closed in case Cookies.sqlite got corrupted.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory: Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox

It is a little confusing that Firefox combines cookies and "local storage" information into the same dialog:

The Cookies column shows the number of cookies, and the Storage column shows the amount of disk space used by storage files for the site. The difficulty in my experience is that even if the storage file contents are cleared, the file might still sit there waiting for future use, taking up space.

I haven't figured out how to tell whether there is actually anything in local storage from this dialog, or whether the file is empty. But I definitely can tell when there are no cookies for the site because that column will be blank.

Note that cookies can't be that big (64 KB), so this isn't a cookie issue, but like posted above has to do with local storage (storage folder in the main profile folder). Local storage and cookies share the same permission (exception) settings.

You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the about:profiles page.

• Help -> Troubleshooting Information -> Profile Directory:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• http://kb.mozillazine.org/Profile_folder_-_Firefox