Open about:config
Now copy and paste following string in Search filter box:
browser.urlbar.disableExtendForTests
Since the preference doesn't exist, Firefox will ask you to create the preference, Select Boolean (it's selected by default) and click on "+" button to add the preference to Firefox.
Now type richresults in Search filter box and look for following preference in the window:
browser.urlbar.maxRichResults
The preference value is set to 10. Double-click on the preference or click on the Edit (pencil) icon to change the value.
Set the value to 0 and press Enter key.

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