Due to its market share, Chromium has better website compatibility these days than Firefox. See the statement by the Brave creator on this: https://twitter.com/BrendanEich/status/1165348116398104576

Also, especially on mobile, Firefox is an extremely niche browser engine. The biggest browser forks in therms of global user count are actually not the likes of Edge, Brave, etc, but android Chromium forks popular in asia.