Assuming you're using Windows, have you tried selecting the first message and then pressing Ctrl + A?

Then, of course, hit the Delete button.

But note: If this is the Sent in your account folders, and you're on IMAP, all the deleted messages will go directly into your account's Trash folder.

I'd go the faster route: Go online to the Web interface of your mail server, open the Sent folder, and empty the folder there.

thank you and no i have not tried the A thing but will

It's an old Windows shortcut for deleting or moving a bunch of files at once. Ctrl + A is often called "Select All," and it works wonderfully when you've got a lot of housekeeping to do.

But do remember that if you fill up the Trash folder on your computer's Thunderbird client AND on your IMAP mail handler, you ought to try to remember to do the housekeeping there, too, once in a while. Many (most) mail servers will permanently (irreversibly) delete all of your Trash that's over 30 days old. You can beat them to the punch by checking your Trash periodically, and moving any items you want to keep for possible future reference to a storage folder (like an Archives folder in Local Folders in Thunderbird) where noooobody can mess with 'em.

I clean sent and trash every day

Mary,

Good For You! If everybody was as diligent as you are, tech support people would have a lot fewer problems with computers that have bogged down to a crawl because they are so full of junk, junk, junk.

Take care. I hope you will use the Ctrl-A trick to clean folders out from now on.

Dan