Do you get a purple shield in the location bar ?

Firefox shows a purple shield instead of a gray shield at the left end of the location/address bar in case Enhanced Tracking Protection is blocking content.
You can click shield icon for more detail and possibly disable the protection.
You can check the Web Console for messages about blocked content.

• https://support.mozilla.org/en-US/kb/enhanced-tracking-protection-firefox-desktop

Yes, that's disabled as well. Still can't log in.

Looks like Firefox is blocking the cookies based on the web console.

Do you have content blocking extensions that could be causing this ?

If you use extensions ("3-bar" menu button or Tools -> Add-ons -> Extensions) that can block content (Adblock Plus, NoScript, Disconnect, Ghostery, Privacy Badger, uBlock Origin) always make sure such extensions do not block content.

• make sure your extensions and filters are updated to the latest version
• https://support.mozilla.org/en-US/kb/troubleshoot-extensions-themes-to-fix-problems

Nope, no extensions that would block content. Currently the browser has no extensions.

I'm on Firefox beta channel (version 80 now), so it's more than latest?

I found the answer to this by looking through the web console errors.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie/SameSite

I set this value to False - network.cookie.sameSite.laxByDefault

Although errors are still being thrown, but I can now log in to the site.