Right-click on Firefox's toolbar then click "Customize". Then in the Customize Toolbar window, scroll and find the "Back" icon and left-click and drag it to the Toolbar (you can position it where you like) then click "Done" when finished.

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

• If items are missing then open the Customize window via "View > Toolbars > Customize" or via "Firefox > Options > Toolbar Layout" (Linux, Windows)
• If a missing item is in the toolbar palette then drag it 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.

See also:

• http://kb.mozillazine.org/Toolbar_customization
• https://support.mozilla.com/kb/Back+and+forward+or+other+toolbar+items+are+missing