Brendan Eich, the founder of Firefox, left the Mozilla Corporation in 2014 and started working on a new browser called Brave. One of the main reasons why Eich created Brave is because he was dissatisfied with the current state of the advertising industry on the internet. He believed that the current advertising model was broken and that users were being exploited by the companies that were tracking their behavior online and using that information to serve them targeted ads.
Brave was created to address this problem by blocking all ads by default and allowing users to selectively choose which ads they want to see. The browser also includes a feature called Brave Rewards, which allows users to earn cryptocurrency (Basic Attention Tokens or BATs) by viewing ads that they have opted-in to see. The idea is that users can then use these tokens to support the websites and content creators that they love, without having to give up their personal information to advertisers.
While Eich could have theoretically worked on improving Firefox, he likely felt that starting from scratch with a new browser would give him more freedom to experiment with new ideas and create a product that was more aligned with his vision for how the internet should work. Additionally, Brave is built on the same open-source Chromium codebase as Google Chrome, which makes it easier for developers to work on and customize the browser.