It's probably on the Customize page. You can normally get that by right-clicking on the 3 bar menu button or a toolbar and selecting Customize Toolbar. If it's there, drag it to where you want it.

https://support.mozilla.org/en-US/kb/customize-firefox-controls-buttons-and-toolbars

If missing items are in the Customize palette then drag them back from the Customize page to the toolbar If you do not see an item on a toolbar and in the Customize palette then click Restore Defaults in the Customize palette window to restore the default toolbar set.