This option may also be found in the App Menu under Options (Windows)
or Preferences (Linux).
Bear in mind that Thunderbird has several windows, and this advice applies to each and all of them, so you must open the offending window first.
Toolbars in Thunderbird can be customised. Open the window where you see the relevant Toolbar. Right-click on the toolbar. If you see a Customise option, select that. If you don't, right-click somewhere else. Some toolbar buttons have their own special right-click actions, so you don't see the usual system menu. Note that the previous View|Toolbars menu shown above also offers a route to this customisation feature.
The window's customisation pane will appear. This has a collection of buttons, spacers and separators that you can drag onto the Toolbar. Whilst
this pane is open, you can also drag buttons about on the toolbar to organise them as you wish. You can also drag unwanted buttons off the toolbar onto the customisation pane.