Are you closing Firefox in between or is Firefox running all the time?

If clearing cookies doesn't work properly 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 when 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/Folder:
  Windows: Show Folder; Linux: Open Directory; Mac: Show in Finder
• https://support.mozilla.org/en-US/kb/Profiles

Thanks very much; that appears to have fixed the issue, though I've only had a couple of days to watch for deleted cookies to reappear. I removed and deleted the cookies.sqlite folder, which took a lot (but not all) cookies with it. I saw the ...-wal folder you refer to, but did not delete it; if my problem reoccurs I will go back and do so. I never saw a ...-shm folder.

Thanks again.

The cookies-shm (Shared Memory) file and the cookies-wal (Write-Ahead Logging) file are temporary files that are normally only there where Firefox is running and should be removed when you close Firefox. It is best to remove them if they are there if Firefox is closed and when you remove the main SQLite file (in this case cookies.sqlite).