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Lars, awesome, you are indeed correct - I was looking for the button in the unified toolbar customizations - but as you noted, it appears on the Spaces Toolbar - excellent, and thank you!

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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