hello, most commonly websites will use cookies to store your login state. in order to control the cookie behaviour in firefox please refer to: Block websites from storing cookies and site data in Firefox

If you keep tabs (windows) open when you close Firefox then cookies used on those tabs are stored in sessionstore.js as part of the session data. You need to make sure to log out on all tabs before closing Firefox.

You can set the browser.sessionstore.privacy_level pref to 2 (never) or 1 (non-HTTPS) on the about:config page to disable saving cookies via session restore in the sessionstore.js file. The browser.sessionstore.privacy_level_deferred pref is used when you do not reopen the previous session automatically via "Show my windows and tabs from last time" and uses the same values.

• http://kb.mozillazine.org/browser.sessionstore.privacy_level

You can open the about:config page via the location/address bar. You can accept the warning and click "I'll be careful" to continue.

• http://kb.mozillazine.org/about:config