What happens if you try to press Ctrl-L (for "Location")?

You can also try to drag the window back into the tab bar of the main window.

(That would be Command + L on Mac)

Make sure that toolbars like the "Navigation Toolbar" and the "Bookmarks Toolbar" are visible:

• "View > Toolbars" (press F10 to display a hidden menu bar)

Open the Customize window to set which toolbar items to display:

• View > Toolbars > Customize

• Check that "Bookmarks Toolbar items" is on the Bookmarks Toolbar
• If the "Bookmarks Toolbar items" is not on the Bookmarks Toolbar then drag it back from the toolbar palette in the customize window to the Bookmarks Toolbar

• If missing items are in the toolbar palette then drag them back from the Customize window on the toolbar
• If you do not see an item on a toolbar and in the toolbar palette then click the "Restore Default Set" button to restore the default toolbar set up

• https://support.mozilla.org/kb/How+to+customize+the+toolbar
• https://support.mozilla.org/kb/Back+and+forward+or+other+toolbar+items+are+missing

• http://kb.mozillazine.org/Toolbar_customization

I have the problem solved since I posted the question. The solution was simple: I went to view, then toolbars and I saw the navigation toolbar had been turned off by another person using my computer. I turned it back on and all was well.