BruceW,
Yes, that led me to the resolution - change the imap server name to imap.ge.xfinity.com.
Seems this issue is discussed in two threads on this forum (This and the one you linked to), on the mozilla forum, and the Ubuntu forum. So apparently it is pretty widespread, but totally off-the radar for Windblows users. (I'm semi-retired from a career in IT, and avoid Windows every chance I get - which is why my personal laptop is Ubuntu.)
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