Without this, you need to select each folder separately to get new emails. This will automatically sync them.

Go to Thunderbird Preferences and search for “Config Editor”. Find the mail.server.default.check_all_folders_for_new setting and double click it to set it to true.

• Ubuntu 20.04.03
• 78.14.0

• https://support.mozilla.org/en-US/kb/config-editor
• https://tech.tiefpunkt.com/2014/08/automatically-sync-all-folders-in-thunderbird/