How about shoving it all in a downloadable plugin which is not bundled with the browser, and to opt-in, it would have to be manually downloaded and installed? Best way to opt-in, and you can even measure how many people actually download it. (Not me,...
One of the main reasons I choose to use Firefox was the privacy first approach. If any privacy threatening, security risk inducing generative AI integration gets embedded in the browser instead maaaazbe as an optional, not installed by default addon,...