https://support.mozilla.org/en-US/kb/websites-say-cookies-are-blocked-unblock-them

Start Firefox using Troubleshoot(Safe) Mode {web link}

A small dialog should appear. Click Start In Troubleshoot(Safe) Mode (not Refresh). Is the problem still there?

I have started Firefox in safe mode and the problem still persists. Plus the "unblock cookies" support link you put didn't help at all.

Start your Computer in safe mode with network support. Then start Firefox. Try Secure websites. Is the problem still there?

Starting Any Computer In Safe Mode (web link) Free Online Encyclopedia

https://www.bleepingcomputer.com/tutorials/how-to-start-windows-in-safe-mode/

https://www.digitalcitizen.life/4-ways-boot-safe-mode-windows-10/ +++++++++++++++++++++++++++++++++++++++++++++++ If the problem goes away with the computer in Safe Mode, the problem is with one of the other programs on the system.

As a test, disable your protection programs.

The problem still persists.

Please provide a public link (no password) that we can check out. No Personal Information Please !

If clearing cookies didn't help then it is possible that the cookies.sqlite file that stores the cookies got corrupted.

Rename (or delete) cookies.sqlite (cookies.sqlite.old) and delete other present cookies files like cookies.sqlite-journal in the Firefox profile folder in case the file cookies.sqlite got corrupted.

https://support.mozilla.org/kb/Deleting+cookies

Type about:support<enter> in the address bar.

Under the page logo on the left side, you will see Application Basics. Under this find Profile Folder. To its right press the button Show Folder. This will open your file browser to the current Firefox profile. Now Close Firefox.

Locate the above file. Then rename or delete it. Restart Firefox.