In addition to all of the arguments brought up by the other commenters against Brave’s blatant lies in this ad, Firefox is deployed in school/corporate environments where compatibility is critical - especially for a web engine that isn’t popular, so they can’t be going around enabling stuff that would break websites on people’s computers in a critical setting, for example during a lesson at school and in front of students.
Mozilla is flawed, but Brave Software is at best just as bad. At least Mozilla isn’t going around spreading absolute lies about competitors.