I found the answer in another post: When NordVPN "Web Protection" is on, it fails to check for updates. This applied in my case and solved the problem.

Thanks for reporting that fixed it for you Damen! As this problem keeps getting reported, I'm going to file a escalated bug about this to light a fire under Nord to get them moving to fix this annoying issue. As they've known about it since Feb 27th & haven't reached back out to Mozilla to investigate & fix this.

Thank you 0.damen for reporting the issue and NoahSUMO for following up with Nord. I looked at the Nord VPN Web Protection settings and Its not obvious to me what sub-settings (Malware Blocker, Web Tracker Blocker, AD Blocker, and URL Trimmer) if any need to be altered or deactivated. I too will inform Nord of the issue and maybe that will light a fire under them. Cheers!

Hi

We believe that NordVPN have fixed their Web Protection feature and you should find that if you turn this back on, the issue is now resolved.

If you find that it is still not working correctly, please let us know and we can try to follow up with NordVPN.

Thank you.

Thanks Paul, you are correct, updating Firefox now works with NordVPN threat protection on.