>>358
thunderbird does not show message - Google 検索
https://www.google.com/search?q=thunderbird+does+not+show+message

からの、最初にヒットするこれじゃね？

Thunderbird inbox has messages, but they don't show on the screen. | Thunderbird Support Forum | Mozilla Support
https://support.mozilla.org/en-US/questions/1423535