Thanks, Fellfrosch.

.qmf, who would have thunk? What's qmf anyway? I looked under .cache, .config, .local, even .mozilla. Never occurred to me to look under .qmf.

You may be pleased to know that I located the "offending" emails and... they indeed do contain duplicate "From:" headers. To be fair, I do not know whether this is a regression or whether it has always been like that. It is not a use case I follow very often.

EDIT: reported in TJC.