Press and hold the CTRL key on your computer keyboard. Click on the emails you want to delete one after the other to select them. Press the delete key. Or click on the first in the list, hold the shift key click on the last to select everything between the clicks. Or click into the list and select Ctrl+A to select everything in the list. With care you can fine tune a select all or select between by changing to the Ctrl key and clicking a selected item to deselect it. Clicking again will re-select This is not really a Thunderbird "function" as the shortcuts have existed in windows since at least version 3, about 1985. I am basing this on using the list or what is now the table view to manage mail as that is how I use it, but a quick check in the newer card view offers the same functionality so you should get the same result whichever you use.