Edit: Actually, I don't have browser.urlbar.update2.engineAliasRefresh configured at all (neither true nor false, just not configured). But for me I've always

Edit: Actually, I don't have browser.urlbar.update2.engineAliasRefresh configured at all (neither true nor false, just not configured). But for me I've always been able to do it. All I had to do was to simply visit my SearXNG instance once, then it would pick it up (OpenSearch API) and then I could just set it as default. In fact I remember setting up my new tablet a few months ago, and I didn't need to mess with about:config at all, it worked like I mentioned above. On firefox mobile they make it super hard editing about:config for some reason so I'm sure I didn't do that.