Firefox HSTs klar

    Close all open Firefox windows.
    Open the browsing history by pressing Ctrl + Shift + H (Cmd + Shift + H on Mac)
    Go to the site for which you want to clear HSTS settings.
    Now right-click on that site and then click on Forget About This Site. Keep in mind that this will clear all data of the site present in Firefox.
Blue-eyed Bison