Rated 3 out of 5 stars

A good idea, but unfortunately as TB has made access to the unified toolbar in TB 115 very difficult, it appears not to work for TB 115.

Hi Rick,
I daily drive Thunderbird 115.2.3 with this extension. It definitely works. Perhaps your installation is configured to hide the toolbar by default. Go to the top right hamburger menu, select View>Toolbars and check "Spaces Toolbar". Then the toolbar with the Proton Mail button will always be shown on the left side of the screen.

Sincerely
Lars