There used to be add-ons that would scan ('crawl') a folder for addresses, but none of them work in 78. The closest may be Copy Address that copies the addresses from a selection of messages to the clipboard.

Yes, I suppose in absence of anything better, this will have to do. Have added it to my TB. It doesn't allow for any columns (From or To) to be selected and harvests every email address in any specific folder.

Thanks though for showing me this extension.