The standard method for restoring the main menu does not work. The main menu is not offered as an option in the custiomize option.
After upgrading from Ubuntu 16.04 to 18.04, I did the usual "trick" of going to the other menu option (top right) selecting "customize" and selecting the "menu bar" option from the "Toolbars" drop down. So far, so good.
This morning the main menu was missing again, but when I tried to restore it, I was not offered the option. I am only presented with the "Bookmarks toolbar" option. Unfortunately I cannot show a screenshot of this behavior, because the selection drop down box disappears as soon as I take the screen shot.
When I press Ctrl-V, the main menu appears in my app-bar at the bottom of the screen, but that's neither permanent nor where I'd like to have it. Oddly enough there is a space between the title bar and the tabs bar where the main menu would normally appear, but in that space there is nothing except on the far right a duplicate of the maximize/minimize/close trio, which are also present in the title bar.
I have tried "restore defaults" and starting from scratch, but that doesn't help either. This is only a problem on my work laptop. My private laptop also has Ubuntu 18.04 and FireFox works as expected there.
Is there some setting I can change in some config file to get the main menu back? Is there anything else I should try?