First off, if you installed it and used it to the point where it showed you an ad, you must have seen the initial screen asking whether you'd like to opt into ads. There's no dark pattern, it's spelled out for you. If you just went "yeah, yeah, whatever", that's on you.
Second, the reason why they have ads is pretty much why the browser itself exists. They introduced a suite of privacy features and an ad model that aims to be privacy-preserving and sustainable (especially in the sense of not letting the web turn into a content cesspool like Google has been doing). In fact their model is the only one I've seen so far that is actually desirable to all actors (advertisers, content producers and content consumers -- yes, including those that don't want any ads, ever). It's a shame it comes with all the stigma crypto has and so it's unlikely to take off.