Does that also happen in Troubleshoot mode?

Yes, it happens in troubleshoot mode too.

Marks them read where? In Thunderbird, or on the server?

In Thunderbird on my Windows 11 PC

I will try again, Check in your webmail before you open Thunderbird. Are the message marked as read

I can see the messages come in as Unread and then immediately change to Read

Do you use the Bitdefender toolbar in Thunderbird?

Yes, I'm using the BitDefender tool bar

Bitdefender toolbar is known to cause your issue. Try to uninstall it https://support.mozilla.org/fr/questions/1447447

I successfully removed the Bitdefender toolbar but the problem is still the same: all emails automatically get marked as read

does anyone else have a solution?

Which Thunderbird theme do you use (Add-ons Manager > Themes . .) ? I remember a long discussion in the German Thunderbird forum about the same problem in version 140.0.1 last year https://www.thunderbird-mail.de/forum/thread/97088-nach-update-ungelesene-email-nicht-mehr-fett/?pageNo=1 We didn't find the cause of the issue and when I saw the user's screen shots there was a bright difference between read und unread message display. The discussion ended with a workaround: https://www.thunderbird-mail.de/forum/thread/97088-nach-update-ungelesene-email-nicht-mehr-fett/?pageNo=4# by using the following CSS code to modify the font in the thread pane

/* thread pane font */
#threadTree tr[is="thread-row"] {

font-family: Palatino !important; }

There also is a second code which can be tested in combination with the upper code

/* thread pane unread colour table view */
#threadTree tr[data-properties~="unread"] {

color: blue !important; font-weight: bold !important; }

If no cause and no solution can be found for your problem you could test the CSS codes: • go to Settings > General >>> sroll to the bottom an click Config Editor.... Search for the preference toolkit.legacyUserProfileCustomizations.stylesheets and toggle its value to true • Open a text editor such as Notepad, copy-paste the above codes in the text editor window and save the file as userChrome.css (attention: remove the .txt suffix your system will add to the file name) • On your desktop create a new folder, name it chrome and move the userChrome.css file in the chrome folder • in TB go to Help > Troubleshooting Information > Profile Folder > Open Folder, then quit TB • move the chrome folder directly in the profile folder, close the profile folder and restart TB Any visual difference now between unread and read messages ?