I had a free hour during BarCamp today, so I started reworking about:config UI for Gecko based apps.
Live demo (for Gecko apps)
And here’s a wiki page for the project (it’s private project, no promise that Flock or Mozilla will want to use this approach ever!)
Comments? Ideas?

Filter input should be much longer in my opinion. Screen wide, to be exact.
I don’t like the way you put so many plain buttons.
I always wanted to see some kind of slider near integer field (when you edit value for property // dbl click). Typing by hand is irritating, using mouse is not.
But nice job, anyway. Present about:config is too cluttered, it’d be nice to see improved version of this mockup in action.
PS: I’d like to see bigger comment field here, now I can’t breath.