Have you ever tried opening a working HTTP website but Firefox keep redirecting it to HTTPS? This happened to me.
I was working on a website and testing a couple of web servers. I was testing Caddy 2 then Apache2 and switching back to Nginx and found out that the domain keep redirecting to HTTPS even when I typed wanted to open HTTP.
How it happen?
I found this blog explained the cause and different approach to solve. Basically in my case, Caddy or perhaps Apache2 injected Strict-Transport-Security into HTTP header value.
Strict-Transport-Security: max-age=31536000
This will force browser to cache HTTPS state for the domain until the time specified expired.
Solution
The solution is quite simple. I have tried many solution which didn’t work until I found this answer. You need to clear site preferences in Firefox settings.
You can either access the shortcut by hitting CTRL + SHIFT + DELETE
or
Go to Preferences > Privacy & Security > History, then click Clear History.
You will then see a popup like this
Make sure all are unchecked except Site Preferences. Set time range to Everything, then hit Clear Now.
You website should be loading on HTTP again.