https means that the website is secured. Unless the site has a non-https link to use . . . . . . .

What kind of problems do you have with that website that you tried to create an exception?

Can you post a link to a publicly accessible page (i.e. no authentication or signing on required)?

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.

I solved the problem by backing up using mozback - extensions, uninstalling FF deleting appdata, rebooting, clean install and and reinstalling mozback backup, and extensions The site now works as it should.

Thank you for your help

That was very good work. Well done. Please flag your last post as Solved Problem so others will know.