That's a positive surprise with the suggestions indeed.
Yes. It's an absolutely terrible UI paradigm for settings, the big example being Windows registry, but also being used in Gnome, Xfce, and FF.I guess you call about:config the registry? I love that feature and wish each piece of software would add that including Windows.
I think it's mostly been a solved problem since around FF15? Or at least it's a solved problem for most popular extensions. Probably the random ones out there doing crazy stuff are the ones that get broken more often.