I want my browser to prevent random extensions from directly reading web page data I also want my browser to prevent random third-party javascript from doing t

I want my browser to prevent random extensions from directly reading web page data I also want my browser to prevent random third-party javascript from doing the same. And I care more about that one, because as a user I don't have control over said third-party javascript while I do have control over the extensions I'm using. The browser is supposed to be a user agent, not act as an extension of the website owner.