(In reply to Andrei Purice from comment #1)
I tried reproducing this issue on the latest versions of Firefox Nightly 85.0a1 (2020-11-24), beta 84.0b4 and release 83.0 on seo.com and storify.com though I am not sure these sites fit the criteria (they do use cloudflare). Can you give us an example of site where this issue is encountered?
Can you test the issue while in Safe Mode? You can find helpful info here : https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-using-safe-mode .
Also a fresh new profile could help. You can find more about creating a new profile here : https://support.mozilla.org/en-US/kb/troubleshoot-and-diagnose-firefox-problems#w_6-create-a-new-firefox-profile .
If possible, you can test this issue on the nightly build as well. Download the build from : https://www.mozilla.org/en-US/firefox/nightly/all/ .
I can't share this specific website, but I'm trying to find others that have this issue.
Anyway I figured out that I can see the HTTPS-only mode select box (see new attached image) only when trying to go to HTTP page that have been upgraded to HTTPS, and I think this is a problem and this select box should be appear always.
But now there is a new problem, as you can see even when HTTPS-only mode turned off for this website, it's still being upgrade to HTTPS. Only when I'm turning of the HTTPS-only mode from preferences, this page stop being upgraded to HTTPS. Why?
These problems exits also in safe mode, fresh profile and nightly build.