Brave has native ad-block and anti finger-printing, enabled by default, and the web works excellently as in I don't know of a single site that doesn't work with it.
Not saying your wrong, but the number, and type, of sites you browse is going to be a very narrow slice of the sites that all Firefox users browse
Brave is based on Chromium and inherits basically all compability, that this brings.
Firefox with Gecko is a different engine entirely and that sometimes causes compatibility issues or different behaviour, that websites have to account for.