In this case I show you two extensions that are very useful to me (separately) since they help me gain vertical space on the screen with my Mail Client. The objective of both is to remove or hide the menu bar from its place, since at least I hardly ever use it and it is taking up space unnecessarily.
I love this extension, especially since it can be customized. What it does is put the menu bar in a unified button, in the style Firefox. In addition, you can change the icon and set other preferences. Look at the following image, how I put the menu with the icon of Thunderbird in the upper right:
I used this extension for a long time with Firefox and what it does is, hide the menu bar, which we can access by pressing the key [Old]. Whatever the case, both extensions strictly fulfill their function.