My installation of Firefox defaults to plain HTTP when I type a URL into the address bar. No amount of about:config fiddling seems to turn it off.
It is rubbish software, the developers routinely ignore fixing actual bugs in favor of new features, and I wish we had a better alternative that wasn't married to Google.
This works fine for me, so I don’t know what’s causing it to be different for you. The key in about:config is dom.security.https_only_mode and I have that set to “true”.
If you want to set this without using about:config you can go to Settings and search “https” you’ll see “https-only Mode” there and you can turn it on for all windows, private windows oonly or none. There is also an exception list should you want that.
It is rubbish software, the developers routinely ignore fixing actual bugs in favor of new features, and I wish we had a better alternative that wasn't married to Google.