The option for "tabs below the address bar" was removed from Firefox a long time ago. Have you been using a "retro" extension, such as the Classic Theme Restorer?

If so, is that extension enabled in the Add-ons Manager > Extensions 'tab'?

I am sorry, I am new to add-ons. I have installed Classic Theme Restorer but I don't know how to make it work. Is it just supposed to appear?

In Firefox, "tabs" generally refers to the different tabs along the top of the toolbar area, indicating different pages you have open in that window. Before Below the address bar, you normally would find the Bookmarks Toolbar. Could that be what went missing? Let's check that first.

To turn different bars on and off, you can use one of these methods to show the toolbar list and select the desired bars there:

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

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

If you hit a link that opens a "styled" popup window, you might find it's not possible to show certain bars there, possibly just an abbreviated address bar. In that case, launch a new window (Ctrl+n) which should have all your standard Firefox features.