There's apparently no way to automatically delete all cookies whenever I quit Firefox. Apparently, it can't always be done manually, either. I have the following in Edit menu > Preferences > Privacy > History: Firefox Will: Use Custom Settings For History Always Use Private Browsing Mode: uncheckmarked Remember My Browsing and Download History: uncheckmarked Remember Search and Form History: uncheckmarked Accept Cookies From Sites: checkmarked Accept Third-Party Cookies: Always Keep Until: I Close Firefox Clear History When Firefox Closes: checkmarked I had two cookies from google.com (PREF and NID). I quit Firefox and restarted it. I did not visit any Google domain but still I had both google.com cookies. I clicked Show Cookies, showing the Cookies dialog, listing the google.com cookies, and clicked Remove All, which, according to the dialog, succeeded. I closed the dialog, quit Firefox, restarted Firefox with the result being the only tab showing the Preferences pages, did not visit any website, and found the google.com cookies were still present. I repeated except that this time I selected each cookie in turn and clicked Remove Selected for each, resulting in the dialog listing no cookies. Again, they came back. And both are listed in the Cookies dialog as "Expires: At end of session". I repeated except that after quitting Firefox I restarted the computer. Still, both cookies came back. I tried Never Remember History. This changed other settings. I selected each Google cookie and clicked Remove Selected, which, according to the dialog, succeeded. In the dialog, I clicked Close. Then I clicked Show Cookies. The cookies were back. Along the way, early on in this test series, I clicked Settings, producing the Settings for Clearing History dialog, and checkmarked everything. It makes no difference. I read <https://support.mozilla.org/en-US/kb/delete-browsing-search-download-history-firefox?redirectlocale=en-US&as=u&redirectslug=Clear+Recent+History&utm_source=inproduct#w_how-do-i-make-firefox-clear-my-history-automatically>, which shows all of the History options checkmarked but none of the Data options checkmarked (the only other difference was that I had used the Edit menu rather than the hamburger menu). Those shouldn't make a difference. But then I had 14 cookies from five domains. I clicked Remove All and the dialog disappeared all of the cookies. I clicked Show Cookies again and the dialog was still empty. I quit and restarted Firefox. The Google cookies (only) were back. This is in FF 40.0 for openSuse 13.2 Linux. Please support automatic mass deletion of all cookies at the end of a session, i.e., when Firefox is quit from or earlier.

Related is bug 1201354 (another test with related effect).

The option as it exists is working. The issue is bug 1026538, which is why there's a google.com cookie that "keeps coming back" and that removal doesn't seem to work on.