Make sure that you do not run Firefox in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History). *https://support.mozilla.org/kb/Private+Browsing *Tools > Options > Privacy > Firefox will: "Use custom settings for history" *Deselect: [ ] "Always use Private Browsing mode" ---- You can rename (or delete) the cookies.sqlite file in the Firefox profile folder to remove all cookies. You can use this button to go to the current Firefox profile folder: *Help > Troubleshooting Information > Profile Directory: Show Folder (Linux: Open Directory; Mac: Show in Finder) *http://kb.mozillazine.org/Profile_folder_-_Firefox ---- You may have installed an extension that maintains a set of OPT-OUT cookies if you can't remove some cookies permanently. *https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes