Firefox

Custom Search Engines: Firefox now supports adding even more search engines. To add a custom engine, right-click a search field of a supported website and select “Add Search Engine”, or go to Settings > Search > Add (below the search shortcuts table) to manually enter a search URL.

"Settings > Search > Add" was already available in Firefox for Desktop, but you manually had to travel to about.config to add the missing browser.urlbar.update2.engineAliasRefresh boolean and set it to true. You've been able to do this for at least 4 years, and it's worked on Android for a while too.

Maybe they finally finished polishing it up.