Firefox only opening http pages in private mode - https alway mode off

Hi I have a couple of http only pages behind a VPN in a private network. When using Firefox for Android and entering the address, either with "http://" or no prefix, Firefox is not able to connect and instantly switches to "https://" mode. "HTTPS Only" mode is not activated, DNS-Over-TLS is also deactivated. The page can be opened when using the IP address. In private mode Firefox opens the page as expected. All other apps on Android do not show this behaviour, i.e. Chrome for example opens the pages fine as expected. With Firefox on Linux, it works fine. How can I make sure that Firefox openes the HTTP page and not switch to HTTPS. About:config is disabled in Firefox on Android so all tweaks changing the settings there do not work. Tried this with and without plugins, no difference.