I've wanted to remove the feature for 8+ years.
I think we should probably remove the feature, and have someone write an extension similar to regclean/regmaid
our extension should basically tell users when a preference name/value became obsolete and let them choose to remove it. RegMaid is the advanced version of RegClean - RegClean or a mozilla equivalent would merely create a backup of the removed prefs and remove all prefs which don't make sense to the current run. It should warn the user about any prefs which are for extensions which aren't enabled.