Cannot delete messages
Performed solution below solution and deleted messages return after a few seconds https://support.mozilla.org/en-US/kb/cannot-delete-messages
Wšykne wótegrona (3)
Who is your email provider? Is this an IMAP-connected account?
My email provider is AT&T
It's possible your mail files are corrupted.
Try to rebuild the index file of the troubled folder. Right-click the folder - Properties - Repair Folder
Note: depending on the corruption this may erase messages from the affected folder which cannot be recovered anymore. In that case you'd need to restore them from a recent backup.