Lordy lordy, why would you want to empty that folder? Don't you feel any need to keep a record of what you said? The developers must have felt something like I do, and didn't think it a use case that anyone would employ.

In your Sent folder, or for that matter any other folder, open the folder, type ctrl+a to select all its contents, then the delete key will nuke them. Alternatively, any other action you might want to do (for example, copy, move, mark as read and so forth) can be applied to the selected messages.

Your explanation is the very reason why there should be a simple option for those of us who don`t want to store "Sent" emails. Maybe commercial users want evidence but everyday home users don`t need to collect sent emails.

SO I must assume that there is a request for an "empty {folder name} menu entry There is no reason to keep received emails any more than there is a reason to save sent mail. As far a home users not wanting a copy of their sent mail, your assertion you emailed some provider of services and cancelled will not get you out of paying the bill. Having evidence that you did it might make all the difference.

But seriously, if you do not want to keep them, why make a copy in the first place? In account setting under Copies and Folders are many options of what to do when you send a message, including the option to deselect the "place a copy in", so not copy is made by Thunderbird. Just be sure you do not use Gmail or any of the Hotmail variants (MSN,Live,Hotmail Outlook and probably a few I have forgotten) as those will add a copy of outgoing mail to the sent folder and offer NO WAY to opt out. Other provides like GMX do not.

"My explanation"? You think ctrl+a, del is too complicated, as opposed to right-click|empty?

Right click|empty would do one thing and only that thing. ctrl+a is a gateway to many alternative actions, so is rather more flexible and powerful.