There is no toolbar button. Either select File-Empty Trash or right click-Empty Trash

Thank you for your response. I was afraid that would be the answer. Does anyone know if there is an add-on or extension that provides this icon/function?

You can create an Empty Trash button here:

https://codefisher.org/toolbar_button/toolbar_button_maker/en-US/messenger/

After you download the xpi installer file, add it to TB by dragging it onto Tools/Add-ons/Extensions. After restarting TB, right-click the Menu Bar, Customize, and drag the button onto the toolbar (see picture).

I had trouble downloading from this site yesterday, but today it worked fine and the Empty Trash icon is installed and working.

Many thanks for your help.