Attaching image since duplicate was noted due to failed upload of image

Make sure that you close all open tabs before closing Firefox to prevent Firefox from storing the cookies in sessionstore.js as part of the session data.

See also:

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

If clearing cookies didn't help then it is possible that the cookies.sqlite file that stores the cookies got corrupted.

• Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete other present cookies files like cookies.sqlite-journal in the Firefox profile folder in case the file cookies.sqlite got corrupted.
• http://kb.mozillazine.org/Cookies
• https://support.mozilla.org/kb/Deleting+cookies

Start Firefox in Safe Mode to check if one of the extensions (Firefox/Tools > Add-ons > Extensions) or if hardware acceleration is causing the problem.

• Switch to the DEFAULT theme: Firefox/Tools > Add-ons > Appearance
• Do NOT click the Reset button on the Safe Mode start window

• https://support.mozilla.org/kb/Safe+Mode
• https://support.mozilla.org/kb/Troubleshooting+extensions+and+themes

cor-el said

Make sure that you close all open tabs before closing Firefox to prevent Firefox from storing the cookies in sessionstore.js as part of the session data.

Is this a new requirement for 36? I have not previously had to close all the tabs for this option to work. This would seem to be contrary to the option to Show my Windows and Tabs from last time when Firefox starts.

I'm assuming Safe mode disables all the extensions. Should it also delete cookies on exit? I.e., if I repeatedly open and close Firefox in safe mode, this would eliminate all interference by add-ons (I don't use other themes). and remove cookies on close?

So far manually deleting them has been the only way.

Installing 35.0.1 resolved this problem (all other settings/add-ons being equal).