I guess it is the address bar that deleted the search engine bar from, not the tool bar.

If you delete an item from the Navigation Toolbar via the right-click context menu then you move that item to the Customize palette, so all you have to do is to click Customize at the bottom of the "3-bar" Firefox menu button drop-down list and drag the search bar back onto the Navigation Toolbar.

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

Thanks so much, it's back.

You're welcome