Thanks. I did some searching and I wasn't the only one to get this problem. I use a cookie manager and can't see any cookies I could associate with Netgate, but I'll check my firewall logs for outgoing traffic. I don't exactly know what FF means by 'Private Browsing', but my Chrome has all its leaky vanilla defaults and that didn't provoke the same login error.
This thread didn't seem to get much discussion, but I'll give his Clone*illa method a try although I expect it will be slow writing to a USB flash drive, although my Pfsense isn't huge. IMHO the only safe backup/restore after a disc, OS, or mini pc crash is a sector copy image. When you have used Microsoft OS as long as I have you learn.
https://forum.netgate.com/topic/97141/dirty-how-to-cloning-pfsense-hard-disk/2
I'm reassured my previous xml settings backup worked for this update, but might no assume the same miracle for 2.5.* Using a data file as a recovery source won't work if the disc format or files it was written for are incompatible. Maybe one day an api to clone a Pfsense box to an external drive and restore from it could be built in? A small part of a recovery routine can sit on a hosts protected partition (which a fresh install won't reformat!) whilst the saved image and the rest on an external usb drive, flash or other?
Thanks again for your help.