Are your Thunderbird folders taking up a lot of disk space, even though you only have a few messages? Is Thunderbird slow to open folders? It’s probably because you have not compacted your folders.
When you delete a message, it doesn’t really get deleted from the folder. It gets marked as deleted. Thunderbird sees that marking, and knows not to display the message. Compacting a folder will command Thunderbird to remove all messages marked as deleted from that folder.
To compact your folders, go to the File menu and select Compact Folders.
To compact an individual folder, right-click on the folder and select Compact.
If you’re having any issues, post in the Thunderbird Support forum at https://support.mozillamessaging.com/en-US/kb/ask