User Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_1) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/78.0.3904.108 Safari/537.36

Steps to reproduce:

I clicked "Get Messages" and received new emails from a Pop3 server. Similar results when I marked an existing message as Unread. Version 60.9.1 (64-bit) Mac-OS 10.15.1 "Catalina" fully updated.

Actual results:

Email messages in message list were shown in regular font

Expected results:

The new and unread messages should have been rendered in Bold font. Previous MacOS and Thunderbird versions rendered unread messages in message list entries in Bold font.

Same problem here with a new but fully updated Catalina install, was working as expected before the OS upgrade.

I can confirm as well.

This appears to be fixed in 72.0b1.

Fixed in 72.0b1, well, I hope much earlier since we suggested update to TB 68 in bug 1563948.

In case anyone would like to solve this in version 60.x.x, this can be worked around by modifying the chrome/userChrome.css file in your Thunderbird profile directory (typically located at ~/Library/Thunderbird/Profiles/<something>.default/ on MacOS, or similar on other platforms).

The following snippet works for me

#threadTree treechildren::-moz-tree-cell-text(unread) {
    -moz-appearance: none !important;
    font-family: Arial;
    font-weight: bold;
}

The key part I think is to indicate a font (Arial here), that MacOS will recognize and know how to display in bold. You might need to tweak this depending on your other settings.

Tested on MacOS 10.15.6 (Catalina), Thunderbird 60.9.1.