Is it a session token stored in a cookie the site uses to connect your request with a running session? I think you have a few choices: (1) Sign out of your session on the site to invalidate the saved token -- this is a good idea whether you then clear the cookie immediately or not (2) Use the site in a private window, then close ALL private windows to empty the private window cookie jar (3) Before leaving the page, manually clear cookies and site data for the site (or if you only want to remove one cookie, use the Storage Manager) -- details below (4) Use an add-on that watches for when all of a site's tabs have been closed and then clears that site's cookies '''Clearing All Cookies and Site Data for One Site''' While viewing a page on the site, click the lock icon at the left end of the address bar. After a moment, a "Clear Cookies and Site Data" button should appear at the bottom. Go ahead and click that. In the dialog that opens, you will see one or more matches to the current address so you can remove the site's cookies individually without affecting other sites. '''''Clearing Individual Cookies Instead''''' While viewing a page on the site, press Shift+F9 to open the Storage Manager. By default, the panel should select the Cookies category but if that doesn't happen, you can select it yourself. Use the right-click context menu to clear individual cookies, or all cookies for the site.