Most probably, you have accidentally turned the Navigation toolbar off.

See this article (the part: I've tried it and I want to go bacl) for steps to activate it again:

Display the Firefox button menu instead of the menu toolbar

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

• Firefox menu button > Options
• View > Toolbars (press F10 to display the menu bar)
• Right-click empty toolbar area

Use Toolbar Layout (Customize) to open the Customize window and set which toolbar items to display.

• check that "Bookmarks Toolbar items" is on the Bookmarks Toolbar
• if "Bookmarks Toolbar items" is not on the Bookmarks Toolbar then drag it back from the toolbar palette into 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 setup

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