I have recently seen this problem after update to 140 in three forums https://support.mozilla.org/en-US/questions/1521800 https://www.thunderbird-mail.de/forum/thread/97088-nach-update-ungelesene-email-nicht-mehr-fett/ https://forums.mozfr.org/viewtopic.php?p=968542 The Bitdefender toolbar is known to provoke this anomaly but it seems that it is not the culprit this time. In a very long discussion in the German forum we didn't find any cause, neither a solution (except for a CSS code to colorise unread messages to distinguish them from read messages).