I only implemented this to deal with preferences that is checked when a file is loaded, since I didn't really foresee us needing much more in the Firefox versions. (Since both the built-in version and the addon uses `about:config` to store the preferences, we need tight control of what is/is not allowed to do.)

The promises was implemented at the suggestion of @brendandahl (but I agree with that idea).

I do think that one of the developers, in this case @brendandahl, should be the one that decides *if* this should be changed, and in that case how it should look instead!