If you have visited this website (domain) in the past via a secure https connection then Firefox may prevail this and use a https connection by default for this domain. 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 (Show All History or History sidebar) or via the about:permissions page. Using "Forget About This Site" will remove all data stored in Firefox from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious and if you have a password or other data from that domain that you do not want to lose then make a note of those passwords and bookmarks. You can't recover from that 'forget' unless you have a backup of the affected files. It doesn't have any lasting effect, so if you revisit such a 'forgotten' website then data from that website will be saved once again.