I suggest also changing something else, anything really. My concern is that thunderbird may not be saving settings when shut down. Tick or untick something, shut down an restart and see if that change took. If not, something may be interfering with shutdown. That might be an antivirus software, or VPN. You can get some basic info by doing this: - click help>troubleshootinginformation - scroll to 'profile folder' and click 'open folder' - now, you are in the profile. If thunderbird is working properly, there will be one prefs.js. If you see such as prefs-1.js, prefs-2.js, etc, that is a sign of the problem. If you do not, then check whether prefs.js read-only. Let us know. thanks.

Prefs.js is not read only.

I'm able to change other settings in the application.