To disable http → https redirects, go to about:config page and set to false the following parameters:
browser.fixup.fallback-to-https = false
network.stricttransportsecurity.preloadlist = false
dom.security.https_first = false
Also check Settings → Privacy & Security → HTTPS-Only Mode is disabled.
See: https://www.reddit.com/r/firefox/comments/1g8lekv/disable_https_redirect/
