Try changing secure DNS over HTTPS (DoH) to "Off" or "Max Protection" and in Connection Settings change "Use system proxy settings" to "No proxy".

Thanks for that.

Unfortunately it does not change the error message.

Secure DNS over HTTPS (DoH) = OFF and = Max protection; tried both - no change.

Use system proxy settings = No proxy; still no change.

It was fine before as far as I know and seems only to affect my login at Tesco.com from Firefox. I can access the site fine, but entering username and password to sign in throws the error.

We had an internet outage for several hours yesterday (TalkTalk Broadband). Is this a coincidence?

Any more suggestions anyone?

Maybe I did not clear things as well as I thought.

Even though cookies and history were cleared and FireFox was newly installed, the Bookmarks were all still there.

I realised that the username and password were being entered for me as soon as I typed the first characters of my username.

I deleted the existing Tesco.com bookmark and entered their site directly from FireFox. It worked fine - no problem accessing my account.

I saved the link to tesco.com as a bookmark exactly as it was before, and that now works fine as well. I can enter my username and password with no problem now, so presumbly there was some corruption to the Bookmark that was stored.

Hopefully this may help someone else as well.

Thanks for the help offered.