I don't know of any way to delete them all in one fell swoop, but there is an add-on that places a delete button next to each address, which is easier to use than repeatedly doing select-and-delete. You just keep clicking on the little X by the topmost address.

https://addons.mozilla.org/en-US/thunderbird/addon/address-close-button/?src=api

Derrell said

When using edit as new on a mail, the previous recipients are listed in the address field. How do I clear those addresses all at once rather than having to delete them one at a time?

Solution works great! A time saver, Thank you.