Well, there’s should be another package that integrates Firefox with the global menu. In Libreoffice there are separate packages that integrates the menu with KDE/QT and Gnome/GTK+ separately.
Firstly, check the global app menu integration in about:config page in Nightly. Click “I accept the risk!” to continue. Search for ui.use_unity_menubar .
If you found it and the setting is turned off, toggle the setting to enable it.
If you do not found it, create/add by right click, select New, then select Boolean. Set the name to ui.use_unity_menubar and set the value to True.
If you uninstall the stable Firefox package for Nightly, check the packages required by reinstalling it. Type in Terminal “sudo apt-get install firefox” (without quotes, requires root access or password). Wait until the terminal asks to confirm the installation. DO NOT confirm and/or pressing Enter, while take the screenshot of the terminal (Print screen button or Screenshots program). Save the screenshot and share it here.
After that, you can abort the installation by pressing N + Enter. Later, I will check what package is needed for app nemu integration.