is there a good list of those prefs? or perhaps I can scan source code for some string
Frankly, these secret prefs are basically either:
- 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.
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.