Display Only Unread Messages in Thunderbird
Follow these steps to see only unread mail in Mozilla Thunderbird:
Alternate Method to Show Only Unread Messages
You can also show unread messages via the following method.
Other Available Options in the View Drop-Down Menu
Using the View drop-down menu, you can also select Not Deleted mail and filter for mail that you have tagged Important, Work, Personal, To Do, or Later. The custom views you can select are:
People I Know Recent Mail Last 5 Days Not Junk Has Attachments
Select Unread Folders
You can also read unread messages in Thunderbird by clicking View in the menu bar and selecting Folders > Unread. This setting shows the folders that contain unread messages, but it shows the entire content of those folders, not only the unread messages.