In Firefox if you press alt the app menu should appear temporarily inside the application window.
Both in Firefox and Thunderbird if you right click on the empty space in the tab bar and check the box next to "Menu Bar" the app menu should stay permanently inside the application window.
In official GNOME applications like Files, Document Viewer, Image Viewer etc. the menu bar is discarded permanently, you'll have to click on their name in the top bar to show the drop-down menu (known as App Menu). For those apps it shows more options like Preferences, Help, About etc., not just "Quit" as in Firefox or Thunderbird.