The best thing about FF is about:config. It tunes in to the legacy of Netscape and Amiga where you could and can configure everything to how you like it. This is freedom, unlike chromium.
If you know about them, for example from reading the code, you can create them. So at present about:config is a strange sort of place with lots of knobs but not all of them.
- prefs that a developer was too lazy to add to the list of pref; or - prefs that serve only during automated tests.
You could scan source code for `Services.prefs` and `nsIPreferenceService` if you really want to find them.