Note: about:customizing is no longer present in Firefox v47+ releases.

• Press Alt or F10 to bring up the Menu Bar.
  Then View > Toolbars > Customize.
• Right-click on a blank spot in the toolbar and select Customize.

Look for Show / Hide Toolbars at the bottom left.

There is also a Restore Defaults button at the bottom middle.

Make sure you do not run Firefox in permanent Private Browsing mode (Always use Private Browsing mode; Never Remember History).

• https://support.mozilla.org/en-US/kb/private-browsing-use-firefox-without-history
• https://support.mozilla.org/en-US/kb/common-myths-about-private-browsing

• Options/Preferences -> Privacy & Security
  Firefox will: "Use custom settings for history"
• remove checkmark: [ ] "Always use Private Browsing mode"