At the bottom of the "Customize" window for dragging and dropping items to and from the tool bar, there is an option to "Add New Toolbar". Clicking on it opens a small window box to name the new toolbar, which I did, clicked "Ok", the box closed and Thunderbird jumped but no bar appeared, either at the top of Thunderbird or as a menu option.. going through the process again using the same name for the new toolbar results in a "Toolbar already exists" message.