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.
According to the comment above, added functionality that's not present in the original like adult sites. It was based off Tachy and now just moved to Mihon as the base.