I'm very much used to having a menu (File / Edit / View / ...) menu at the top of the screen. But Xubuntu neither shows me a menu at the top of the individual window, nor one at the top of the screen. Here's what it looks like now:
In contrast, GIMP and a few other applications have them:
What can I do to restore them?
Firefox has such menu, but it is hidden by default to make more room for the actual browsing content. Top menus are part of the old Firefox interface. Firefox is making transition for the new interface type: touch-based and screen-size independent.
You can find Firefox menu on the right side by pressing the button with 3 horizontal lines. Like the one in Android.