How can I remove a website accidentally added to Firefox's list of HSTS sites?

I found additional HSTS data that was not deleted via "Forget About This Site" or clearing the browser cache in a file SiteSecurityServiceState.txt in my profile directory.

It is a text file you can open with any text editor and remove lines of HSTS data about a speficic host. Or clear the whole file.

For the sole reason that I just had the same issue (and not because I really like resurrecting old questions!), you can also open the History browser, find an entry for the site in question, right-click the and select Forget About This Site.

This will remove your history entries for that site only (including the HSTS setting), without affecting your other history settings.

You need to close all tabs for the site that you wish to forget first, otherwise the tab itself seems to remember.

(At least, it did for me, on FF26/Win32...)

Clear all browsing data from Firefox.

Ctrl+Shift+Del shortcut for Windows, or Firefox button -> History -> Clear recent history.