You can remove all data stored in Firefox from a specific domain via "Forget About This Site" in the right-click context menu of an history entry ("History > Show All History" or "View > Sidebar > History"). Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks and history and cookies and passwords and cache and exceptions, so be cautious. If you have a password or other data from that domain that you do not want to lose then make sure to backup this data or make a note. Can't recover from this 'forget' unless you have a backup of involved files. If you revisit a 'forgotten' website then data from that website will be saved once again. ---- Are you using a proxy or use security software that places itself between Firefox and the server on the internet? *https://support.mozilla.org/kb/troubleshoot-SEC_ERROR_UNKNOWN_ISSUER You can check the connection settings: *https://support.mozilla.org/kb/Firefox+cannot+load+websites+but+other+programs+can