UPDATED.

It turns out that you have to add the [W] button to the Bookmarks Toolbar to get this Extension to work. (at least, as of 01-DEC-2021)

Now things are working fine.

ADDENDUM:
Actually, things are working fine as long as the Icon is visible in the Firefox window/toolbar. If I move the button/icon over to the end of the row, we go back to "nothing shows up" status. If I shrink the window so the 'W' icon is no longer visible, we go back to "nothing shows up" status.

If I don't want the definition pop-up in the middle of the window, tough.

Last update broke the "Other Languages" button. Please rollback.