Comment 2 for bug 1890049

While we appreciate this is effectively a functional regression, it is intended. Exposing firefox's menus to the global menu bar involved a non-trivial distro patch that was increasingly difficult to maintain (because it relied on many internal APIs), so it was dropped before 20.04 (see https://bazaar.launchpad.net/~mozillateam/firefox/firefox.focal/revision/1340).

The firefox updates in 16.04 and 18.04 still carry the patch, FWIW.