This is not really true. The about:config flag you mentioned does add a button in the Settings page allowing the addition of custom search engines, but that's only because the intended method for adding search engines is different. The normal way doesn't require setting anything in about:config, but it might be a little hidden (still found it on my own though, so I dunno).
The regular way is to visit the homepage for the search engine you want to add and click the URL bar. This will show the usual search drop-down with the "This time, search with:" row at the bottom. If you're on the homepage for a search engine which hasn't been added to Firefox, there will be an additional button on that row allowing you to add the site as a search engine, which will make it accessible from the regular settings page like every other search engine.
The regular way is to visit the homepage for the search engine you want to add and click the URL bar. This will show the usual search drop-down with the "This time, search with:" row at the bottom. If you're on the homepage for a search engine which hasn't been added to Firefox, there will be an additional button on that row allowing you to add the site as a search engine, which will make it accessible from the regular settings page like every other search engine.