Generally, you can able to restore Firefox default search engines (note if everything is fine, ‘Restore Default Search Engines’ button will be grayed out by default, unless you make changes like removing a search engine from the list, you won’t see that button active) by visiting search preferences. What if you’re using a search engine that came from unknown origin as default? Mozilla wanted to alert you in that case, hence Search Reset feature was planned.
Firefox Search Reset feature planned for 51
At present, this feature is hidden behind a preference –browser.search.reset.enabled– and disabled by default in Firefox 49, when enabled, you can see the feature by visiting about:searchreset page, which asks the user to
‘Restore your search settings? Your search settings might be out-of-date. Firefox can help your restore the default search settings. This will set your default search engine to Google (browser set default search engine in this case). You can change these settings at any time from the settings page’ and the dialog offers ‘Don’t change’ and ‘Change search engine’ buttons.
User who prefers to stay with the current search engine, will keep it, otherwise, clicks the Change search engine button. Mozilla has targeted this feature for Firefox 51.