Go to the Gmail website and deal with the badly behaved message there. Answer it, delete it or move it to some other folder. Or use one of your alternative clients that does handle it successfully.

My guess is that it is malformed such that Thunderbird thinks it is incomplete and there is more to come, so it is left in limbo, and not marked as read or downloaded.

Thank you for the suggestion, but the badly behaved email does not appear anywhere except for my Thunderbird Gmail. I have reached out to the marketer in hopes that they can intervene, but so far, it seems to be a problem with Thunderbird. One very odd thing is that the badly behaved email never did appear via the Gmail website. I have emails from the same marketer before and after September 4, but not on September 4 when this message tripped up Thunderbird.

disable your email scanning in your anti virus product, then complete the download. My information is this was a behavior exhibited by Norton's in the face of certain malformed emails, but other anti virus product could likewise be tripped up by malformed emails.

Thank you for your help! Disabling the email scanning in my antivirus product immediately allowed the bad email and all of those since to come in. I deleted the bad email and re-enabled the email scanning in my antivirus.