First, I would always close everything down, then open Windows, then open Firefox.

If is the whole Navigation Bar that is missing - right click on toolbar space and check Navigation Bar.

If it is just the Address Bar (this will undo any special customization you've done)

Courtesy of TonyE -

To restore it, try the option to "Restore the default toolbar set" as shown here - https://support.mozilla.com/kb/Back+and+forward+or+other+toolbar+items+are+missing

If that does not work, try the option to "Reset toolbars and controls" in the same article.

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