This one is really tricky. You can spam CTRL + F5 all day but firefox will still remember Redirection 301.
You CTRL + H aka History Bar, look for page you want to forget of, Right-Click and pick “Forget about this page” which in Polish is “Usuń całą witrynę” which is “Remove whole site”. This is why I “love” translations.
If this fails because you where redirected to error page or something you can try:
Menu -> Tools -> Options -> Advanced -> Network -> Clear Now