Customizing the toolbar in Thunderbird is a snap. You simply right-click the toolbar, highlight the Customize button, and then drag and drop the icons you want to use to the toolbar area. If you want to get rid of an existing icon in the toolbar, all you have to do is drag it back to the Toolbar window.
If you are looking for some new icons to add to your toolbar, these extensions provide some great options:
This has been a long and fruitful journey. Our journey has taken us along the highways and byways of Thunderbird, demonstrating how you can use Thunderbird to manage your email, how to protect your privacy and security, and how to customize the program using extensions and themes. With Thunderbird, there are many roads to take and many roads to further explore as development continues on this product. Remember, the development community and all the open source contributors continue to make Thunderbird the best it can be.
I conclude with a quote from Lewis Carroll, from his seminal work Alice in Wonderland: "Begin at the beginning and go on till you come to the end; then stop." You can stop reading now, but don't stop having fun experimenting with Thunderbird and what it can do for you.