• 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.

Et voilà.