Maybe "Forget About This Site" (via the right-click context menu of a history item) will help, but that will remove everything from that domain like bookmarks, cookies, passwords, cache, history, and exceptions, so be cautious if you decide to give that a try.

I tried "Forget About This Site" but still cannot access the developing site with HTTP. I guess the HSTS setting is saved in a different place from the history items.

Maybe rename secmod.db and if that still doesn't work cert8.db as well to see if that catches it.

I renamed secmod.db and cert8.db, but still cannot access.

Remove all files in my profiles folder => Cannot access. Private browsing mode => Can access! But I don't know why it works.

I want to access the site without private browsing mode.

Private Browsing bypasses the cache (disk cache is disabled) and gives you a new set of cookies.
So Firefox may be using the cache or data stored in the disk cache.

Did you already try to clear the cache?

• Tools > Options > Advanced > Network > Offline Storage (Cache): "Clear Now"

You may need to use Clear Recent History with the time range set to everything to clear the cache.

• https://support.mozilla.org/kb/Clear+Recent+History

Thanks! Finally I resolved this problem!

Your first reply: "Forget About This Site" (via the right-click context menu of a history item) is the solution, but I guess that once I access the site with HSTS in a browser tab, the tab remembers the HSTS setting after "Forget About This Site".

I tried to reload with F5 key in the same tab, so I cannot access even after "Forget About This Site".

I can access with a new tab now.

Thank you very much, cor-el.

You're welcome