Mozilla, the company that recently added a "disable AI" toggle to Firefox?
It's not as good as not implementing the things in the first place, for people who don't want it, but making it a paid model for removing features would net them more flack, I feel. They'd be accused of trying to squeeze their user base, when Firefox is traditionally free.
Vivaldi still supports Manifest V2, doesn't it? So you should be able to put UBO on it as well.