I’m not sure which build of firefox the other user is using, but in the stable stock build I can’t add it in settings page itself either.
But if you go to a website that can do search, eg. Startpage, then right click on the browser’s url bar again, a new option to add the search engine pops up at the end of the context menu.
Another alternative is installing addons that adds the search engine too, I believe duckduckgo & Startpage has theirs.
Here’s firefox’s docs about it - it’s got screenshots of how it looks.
Edit: Ah, I see you’ve tried it already - probably plugins are the next easiest way to go about it then :( The docs do give a direction though of where to go, like repositories of supported sites.
Try using the Mycroft Project that the page mentioned, it’s got user maintained search engine plugins.