Do you have the Status Bar hidden? It's under View/Toolbars; press Alt if the Menu Bar is hidden.

I checked and status Bar is ok. My opinion is that is due to the upgrade. Everything is working fine but I would appréciate to have this information near my accounts name. Thanks for your reply

If you mean the number of new messages that appears next to the Inbox icon in the Folder Pane, that is still there in TB 78. Do you have the Folder Pane hidden? It's under View/Layout; press Alt if the Menu Bar is hidden. If the Folder Pane is shown but not the number, see if it changes in safe mode (hold Shift when you launch TB).

Hello, I checked in safe mode and the problème is not fixed. What I noticed is that the color of my accounts name change from black to blue when news mails are waiting to be downloaded., but no number to indicate of many. Otherwise everything is working fine with Thunderbird 78. But this missing number can be useful. Thanks for you help

I forgot to mention that the number appears only when the mails have been downloaded on the "Incoming mail" Tab. Thanks

Not sure what you mean that the number appears only after mail has been downloaded. Do you not have TB set to download at startup and periodically, or when mail arrives at the server (IMAP)? Check the Server Settings section of Tools/Account Settings.

Hello, I checked the server settings and it's configure to not load mails automatically, with a period checking of 10 minutes. If I can't resolve this issue it does'nt matter, I'll do it without this information. Thunderbird is working fine. Perharps in a other upgrade it will be resolved. Thanks for your help