Stefan. It is a real nightmare.
I am afraid that none of that works with Firefox. You cannot accept the certificate because it will not let you. I have deleted cookies, cache, Active logins, offline Website data, any Bookmarks relating to Irfanview forum. I also opened About:config, searched for and deleted the reference to Irfanview that I found there. After deleting all history I now can see none at all when I ask to view all history, but it is still somewhere because as soon as I enter the new forum URL a drop down list appears of just about every place on the old forum I have ever visited.

Fortunately Chrome does allow me to come here but I fear we may be about to lose a lot of members unless we can sort this out.