I find it odd that this extension has to exist at all. It adds the "Add New Toolbar" button in the Customize toolbar window from Firefox into Thunderbird.
Some people will be pleased to know they can use this to get the "Location" bar that was in Thunderbird 1.5 back into Thunderbird 2 and 3. If you have many buttons on the menu bar and default toolbar in Thunderbird, this extension lets you create additional custom bars so you can use even more buttons.
This functionality is also included in my "Toolbar Buttons" extension. If you already have that installed, there is no need to install this one too.
Please note that there are no conflicts between TB Custom Toolbar (or Toolbar Buttons) and the xSearchbar extension by Philip Chee. You can have both installed at the same time. Both extensions do slightly different things and work well together.