ctrl+a will select all the messages in your folder. The delete key will do the rest.

if I have 1,000 folders and on each I select all messages. Why ??? If you can just right-click on the folder and be delete all messages.

You want a recursive delete of non-empty folders? Really??

I'm just asking to make a point "Delete all messages" from the context menu.

Drag & drop to trash may also work.

the question is closed.

Not exactly what you are looking for, but you can set a retention policy on an account or on a per folder basis, so it would auto delete eg: anything older than x days. Then you don't even need to perform a manual delete even if it were only clicking on a button.

I'm not saying it is what you want, but it may offer you a solution.

Be aware that both of these methods delete emails from Thunderbird and also off the server.

On account: Tools > Account Settings > Disc Space for the mail account make your selections and click on OK.

Per Folder: Right click on a folder and select 'Properties' click on 'Retention Policy' tab make your selections and click on OK.