I reinstalled empty'em and it hasn't jammed up today.
I've got seven email accounts, all POP configured.
The symptoms I was observing amounted to: empty'em would seem to run
and then later, if I tried to delete a post I'd read, or mark one as
either one would be ignored unless and until I re-started T-Bird.
I've just updated to 6.0.1 and will report any further problems if
I'll also check the error console if the problem recurs.