So you are saying you messages download from the server in a read state? If so do you get your mail using the IMAP protocol? Do you also get it on you phone or a tablet?

I have seen lots of instances where mail appears in Thunderbird as read because the server (gmail in particular) marks mail as read when picked up by the phone. In other cases I have seen folks most excited about the issue only for it to turn out they had purchased an "internet security suite" with scam and phishing detection that truly messed with their mail read state, the folders it is in and if it is junk or trash, but didn't really protect them.

Sometime the folder index is corrupt and doing a repair by right clicking the folder and selecting properties and then the repair button can cause the read state to change.

no they are not read-only. Before updating the MAC to Catalina I saw the messages received in bold. Now I don't see them in bold anymore.

The problem appears to be fixed in TB 68.1.1:

https://bugzilla.mozilla.org/show_bug.cgi?id=1563948

TB 68.1.1 for mac:

https://archive.mozilla.org/pub/thunderbird/releases/68.1.1/mac/

sfhowes said

The problem appears to be fixed in TB 68.1.1: https://bugzilla.mozilla.org/show_bug.cgi?id=1563948

then? I have version 60.9.0. the problem is always there

I had the same problem and I just found a solution! yay. In Thunderbird 60.9 please go to Tools>Add-ons then search for "Theme Font & Size Changer (fixed)" and add it. Then quit and restart Thunderbird. What I think happened is that it got disabled when upgrading to Catalina. Good Luck.

Thanks! That worked! (I had just upgraded to Catalina also....)