Folders in Mozilla Thunderbird can sometimes lose their underlying structure. This can cause archived messages to be lost or previously deleted emails to suddenly reappear. Here's how to repair Thunderbird folders if this happens to you.

To have Thunderbird rebuild the folder index and ensure that it accurately reflects the messages currently in the folder:

Disable automatic email checking in Thunderbird as a precaution. This may not be necessary, but it can prevent potential conflicts.

Right-click on the folder you want to restore and select Properties from the menu that appears.