Thunderbird updated and I noticed a search bar at the very top. I don’t like it. A bit of clicking around and I found out how to remove it.
Right-click on the top bar, somewhere around the search bar, and click on “Customize…”
Then right-click inside the search bar and click on “Remove”. Then down in the right-hand corner, click on “Save”.
If you want it back, click on the “Restore default” button on the bottom left-hand side.