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.
I don't need to be a source code expert to understand what Mozilla has done, thankfully. That's not a technological hurdle the average person should have to overcome. I know more than most people have the privilege to know, and further gatekeeping (any gatekeeping) is unnecessary.
If that was not your intent, or for other people who are genuinely interested:
If gatekeeping my comment was your intent, I would be happy to track down the source code as long as you made an extensive effort to get Mozilla to remove the code that collects your data...
I'm glad you thoroughly understand the concept of "the lesser evil"