You can tap the Alt key or press F10 to show the Menu Bar

Note that you can find "Options/Preferences" in the "3-bar" Firefox menu button drop-down list.

• https://support.mozilla.org/en-US/kb/Options+window

But the top menu disappears again. How do I make it stay there?

It should stay until you have done something. Is the menu bar closing on its own?

Start Firefox in Safe Mode {web link} by holding down the <Shift> (Mac=Options) key, and then starting Firefox.

A small dialog should appear. Click Start In Safe Mode (not Refresh). Is the problem still there?

ingemar23 said

But the top menu disappears again. How do I make it stay there?

To turn the menu bar (or another bar) on or off, you can use one of these methods to show the toolbar list and select the desired bars there:

• right-click (on Mac Ctrl+click) a blank spot on the tab bar (or the "+" button)
• (menu bar) View > Toolbars
  On Windows, tap the Alt key to activate the menu bar temporarily.
• in Customize mode > Show/Hide Toolbars (see: Customize Firefox controls, buttons and toolbars)

To turn on the Menu Bar, Bookmarks Toolbar, or other bars, click it on the list.

Success?

Success! Thank you very much!!