Toolbar customization - Thunderbird
From MozillaZine Knowledge Base
It is very easy to customize the layout of the toolbars in Thunderbird. You can add or remove buttons, and change their placement. The main Thunderbird window, the Compose window, and the Address Book window are individually customizable and have different sets of icons that you can add to or remove from each.
Customizing the toolbar buttons
To customize a toolbar, open the window that you want to customize, and then go to the "View" menu and select "Toolbars -> Customize"; alternatively, right-click on the toolbar and select "Customize". The Customize Toolbar dialog will then pop up. To remove a button, simply drag it from the toolbar down into the Customize Toolbar dialog. To add a button, drag it up from the Customize Toolbar dialog and place it on the toolbar. If you want, you can even put buttons up on the menu toolbar.
Adding buttons provided by extensions
Some Thunderbird extensions (such as the Buttons! extension) provide extra toolbar buttons as part of their functionality. Installing an extension does not automatically put its buttons on the toolbar, however. To add these buttons, you should do as described above for adding buttons to the toolbar.