Mozilla is working on new Preferences for Firefox browser by reorganizing the current one in release version. Like Chrome, which allows to search within settings, the company has added experimental search box to reorganized preferences, currently available via about:config, but disabled by default in Nightly (Firefox 55).
You need to toggle this preference value browser.preferences.search to ‘true‘ to see it in action. Without you need to dig into settings, the search box appear on top right of about:preferences page shows the search results after typing whatever you’re looking for.
h/t to [Soren]