Some Firefox settings can be locked or overridden through external settings files that Firefox reads when it starts up. Could you check the setting in the about:config preferences editor?

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box that appears above the list, type or paste cook and pause while the list is filtered

(3) Look for the network.cookie.cookieBehavior preference and check its value, and whether it says "default" or "user set" or appears in italics and is locked.

• 0 => default: cookies accepted
• 2 => cookies blocked (what I expect when you uncheck the box)
• 1 => main server cookies accepted, all third party cookies blocked
• 3 => main server cookies accepted, third party cookies accepted if you have visited the server directly, otherwise blocked

Anything strange there?

Cookie-management or settings-locking add-ons also could be monitoring/overriding changes to this setting. Your "More system details" didn't list any extensions, but you can review and in some cases configure them (using an Options button) on the Add-ons page. Either:

• Ctrl+Shift+a
• orange Firefox button (or Tools menu) > Add-ons

In the left column, click Extensions.

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

If you do not keep changes after a restart or otherwise have problems with preferences, see:

• http://kb.mozillazine.org/Preferences_not_saved