Does the QR code appear for 1 second then disappear? A user on reddit found setting privacy.resistFingerprinting in about:config to false (from true) corrected this exact issue.

Let me know.

I think that "Resist Fingerprinting" would give you a different user agent (68.0 ESR).

With Tracking Protection enabled I still get the QR code, so that shouldn't cause it as well.

Do you have other content blocking extensions ?

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

Privacy.resistFingerprinting in about:config to false and restart without addons not helped me.