That is not possible on Mac. On Mac the main Menu bar with the "File, Edit, View" and other items is under control of the Mac OS and you can't place other items on it like is possible on Windows an Linux. You can place items like extension buttons on all the other toolbars (Navigation Toolbar, Bookmarks Toolbar, and Tab bar).

• https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars

I probably wasn't clear with my question. I want to place extension icons on the Firefox menu bar, not on the main Mac menu bar. Although I've installed several extensions (add-ons), I don't know how to run them except by using the icons. I didn't have this problem with earlier releases of Firefox or on Mac Mojave OS.