WORKAROUND IDENTIFIED.
PROBABLE SOURCE OF BUG IDENTIFIED.
Using Unity Tweak, I changed the Theme from "High Contrast" to "Ambience". Missing menus are now visible. Change back to Ambience and bug recurs.
The cause of this bug seems to be related to the "High Contrast" Theme.
This workaround also applies to the related bug #1440274 Thunderbird Menus Not Displaying In Top Menu Bar of Windows
WORKAROUND IDENTIFIED.
PROBABLE SOURCE OF BUG IDENTIFIED.
Using Unity Tweak, I changed the Theme from "High Contrast" to "Ambience". Missing menus are now visible. Change back to Ambience and bug recurs.
The cause of this bug seems to be related to the "High Contrast" Theme.
This workaround also applies to the related bug #1440274 Thunderbird Menus Not Displaying In Top Menu Bar of Windows