This usually means that the favicons.sqlite file that stores the cookies is corrupted. There is no way to repair this for only one favicons and you may have to rename or remove favicons.sqlite and in severe cases rename/remove places.sqlite as well. You can check for problems with the <b>places.sqlite</b> and <b>favicons.sqlite</b> files in the Firefox profile folder. *use "Verify Integrity" button on the "Help -> Troubleshooting Information" (about:support) page If errors are reported with "Verify Integrity", close and restart Firefox and retry. If "Verify Integrity" cannot repair places.sqlite, rename/remove all <b>places.sqlite</b> and <b>favicons.sqlite</b> files in the Firefox profile folder. Firefox will rebuild places.sqlite and restore the bookmarks from a recent JSON backup in the bookmarkbackups folder. *keep a backup copy of places.sqlite in case a new places.sqlite database has to be created See also: *https://support.mozilla.org/en-US/kb/Bookmarks+not+saved#w_fix-the-bookmarks-file *https://support.mozilla.org/en-US/kb/fix-bookmarks-and-history-will-not-be-functional You can use the button on the "Help -> Troubleshooting Information" (about:support) page to go to the current Firefox profile folder or use the <b>about:profiles</b> page. *Help -> Troubleshooting Information -> Profile Folder/Directory:<br>Windows: Open Folder; Linux: Open Directory; Mac: Show in Finder *https://support.mozilla.org/en-US/kb/profiles-where-firefox-stores-user-data

Thanks cor-el... however: Task: checkIntegrity + The places.sqlite database is sane + The favicons.sqlite database is sane Are there other things to check or should I still go ahead with the steps you suggested "if errors are reported"?