Try the following:

1. Right click on an empty area on the tab bar
2. Select Customize... from the drop-down
3. Drag the Search item to an available area on a toolbar
4. Press Ok

Please report back soon.

If missing items are in the toolbar palette then drag them 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 setup.

• View > Toolbars > Customize

• https://support.mozilla.org/kb/How+to+customize+the+toolbar
• https://support.mozilla.org/kb/Back+and+forward+or+other+toolbar+items+are+missing