Actually iff it's user-controlled I'd love such a feature. As sibling comment notes, it'd be nice to have zero extensions (or maybe just uBo) running on sensitive sites like a bank, but I'd actually be more interested in running certain extensions only on select sites; ex. there are extensions to force videos to play at arbitrary speeds that I like for video sites, but I want to only let those extensions run on a very short list of sites (youtube, netflix, w/e) and nowhere else.