The answers given here do not match my problem. I accidentally landed on http://sub.domain.org/ which 301 redirected me to https://sub.domain.org/. Then I changed my server config so that it does not redirect me anymore, but Firefox keeps redirecting. The Firefox console log says:
GET http://sub.domain.org/ [HTTP/1.1 301 Moved Permanently 0ms]
GET https://sub.domain.org/
So question is: how to make Firefox forget this "permanent redirection" specifically? I would prefer not delete my whole cache.