You can open about:permissions page in Firefox and you can manage Cookies stored by the websites you visited, you can remove all of them or selectively, of course you can control default permissions for all sites on this page. Cookie Controller is Firefox add-on adds two buttons to manage cookie permissions and Site cookie exceptions.

Cookie Controller

Using Cookie Controller

Download and install the add-on and restart your browser.  Now drag-n- drop two buttons of Cookie Controller to the Add-on Bar by selecting Firefox Menu>Options and Toolbar Layout.

One button, man standing one is the global Cookie setting lets you allow or block all cookies. By default, Firefox set to allow all cookies and discard them at the end of the session. From this button, you can manually add websites you want to block/allow by adding them under Exceptions.

Another button Site Cookie Exceptions one indicates exact cookie permissions for the current page. You can allow cookies for the session, or temporarily or you can deny cookies for that site. You can create an exception for specific site like denying or allowing cookies and this exception can be removed from Global Cookie Permissions button’s exception list for that host.