Whatever was going on, it affected two of my systems - a Win7 and a WinXP, with fully updated Firefox. However, it might have been a Firefox bug, because after tonight's Firefox update, the problem has gone away. Sorry, Jon, might have been me this time. Or, at least, might have been Firefox.
I did a bit of Googling and came across this error with people using Firefox. Maybe it was that. In future, if it happens it would be easy to test another browser to rule it out.
Yep, I still have the "standard" Microsoft browser, though to me it is poor when compared to Firefox. But I will remember to try it next time before sounding the alarm.
When I was still with the Navy as a contractor, we were given a list of approved browsers in order of preference. Firefox was higher on the list (at the time) due to its ability to run the on-line training presentations mandated by the Navy for all affiliated personnel. As I recall, Chrome was also approved but with a note regarding that it would be more likely for the presentations to work with Firefox than with Chrome. By now I would expect that any issues have been resolved, but at the time, I used Firefox and just got accustomed to it.
I used to use Firefox too because it had lots of extensions and plugins back then. Chrome overtook it and so I did the switch. I just cannot get into Microsoft's latest effort, Edge.