The [Restore Default Search Engines] button in Search Options is clickable ONLY if you removed a search engine that comes with Firefox by default (like Google, Bing and Amazon). Norton Safe Search did not come with Firefox so you'll have to add it back yourself.

Ref.: Change your default search settings in Firefox under Remove or add search engines

Thanks,very much, for such precise and prompt answer. Indeed, I had at beginning an option (promotion) to 'obtain' Norton Safe Search, which I did - and it did,initially,appear in the 'one-click' search engine list (at the end, after Wikipedia, I believe). I, inadvertinely deleted it (not understaning expl. what I was doing). Then, via same document URL you kindly included, I realized it is not a Firefox supplied option. Thus, I could (via this document) reobtain it as an 'add-on', and now it appears in the RHS horizontal search field at top of browser LHS: URL field; RHS: search field; but not in one-click search list. My pedantic request is, how can I get it back in the one-click (selection list) again ? FY kind info., i'm a 'medium experience' user of Windows 10. If you have a procedure for this, I'll be musch obliged. Thank you. sincerely, trivan pal.

I don;t know if there's a difference between Norton Safe Search as an "add-on" and Norton Safe Search as a 'one-click' search engine. I found https://us.norton.com/safe-search which has a support link. You can also try the norton community forums at https://community.norton.com/en/forums

I installed Norton Safe Search in a test profile from the [Try Safe Search now] link at https://us.norton.com/safe-search and it added a "Norton Safe" entry to the one-click search engines list. It also installed Norton Safe Search an extension and changed my Home and New Tab page to "Norton Safe Search. I then disabled the Norton Safe Search extension in the Add-ons manager (see Disable or remove add-ons under Disabling extensions). That restored my default Home and New Tab page but it removed the "Norton Safe" entry from the one-click search engines list.

Hope that helps.

Thank you, very much. Your procedure and 'pointers' have exactly solved my issue, which I was a bit vague in making precise. I thank you immensely as well as the Firefox development/support community for this excellent and rapid help! Take care, be safe in these testing times. sincerely, trivan pal.

You're welcome. I'm glad you got it sorted out.