Before it magically started working again after an hour, during the time
when it wasn't working I did try it in safe mode, and that made no
difference (it still gave that error message). At any rate, using the
same profile I've always had (and I only have one), it didn't work
multiple times over the course of an hour, then suddenly started
working. And during the time it wasn't working in Firefox, it did in
IE. But it's working again now.