Your mail server is holding onto those 3000(!) email messages, and T-Bird is just letting you know.
Also, you might have your account set up to leave messages on the server. Thus, your email there never gets expunged from the server.
To check and possibly change that setting:
Select Tools>Account Settings
Select "Server Settings"
Uncheck "Leave messages on server
Why would anybody want to leave the messages on the server?
If a person has multiple computers to access their email on a defined mail server, BUT there is one computer that they mainly use to "keep" all their email, they might set all other computers to leave the email. Then on that one mail computer, they have it set to NOT leave the messages. That way, when eventually using that one computer is how they can read and keep track of all the email coming and going.
I only occasionally use this email facility but when I do it comes up with a little menu at the bottom right hand corner of the page saying "Local Folders has ..... messaages" It has now reached over 3000. How do I access this local folder to delete them.
Grateful for advice