Lost your menubar? Using Xubuntu / XFCE Desktop? Global menu applet might disable the menu inside the app, but not show it at the top of your screen either.
Fix: Just remove ‘indicator-appmenu’ package, log out and in.
First try that you can’t get your menubar back with ctrl-M (Okular) or toolbar context menu (Thunderbird).
Additional symptoms include reappearing “MenuBar=Disabled” in .kde/share/config/okularrc and other settings files.
$ sudo apt-get purge indicator-appmenu
Then log out and back in for settings to take effect.
Start run dialog with F2, “okular”. If menubar is still hidden, press ctrl-M to show it. Now you should see menubar on top of Okular.
I have tested this fix on Xubuntu 18.04 LTS, the xfce4 desktop for Ubuntu. Update: added note about XFCE.