I'm using the current Firefox-ESR for Linux x86_64. Under ''Settings > Privacy and Security > Cookies and Site Data > Manage Exceptions'', I blocked ''http:174.138.51.92'' and ''https:174.138.51.92'' several weeks ago. But ''Manage Data'' shows 10 cookies ''last used 45 minutes ago'', and ''Remove Selection'' for these cookies fails (incidentally retaining the "last used" value), even when repeated multiple times as fast as I can mouse the ''select'', then ''Remove Selection'', then ''Save Changes'' then ''Manage Data'' . How can I delete these cookies and site data? And why does it fail for this site and this site only? And how can I block them permanently, short of black-holing that site in ''/etc/hosts''?