Browser extensions have much looser security than you would think: any extension, even if it just claims to change a style of a website, can see your input type=password fields - it's ludicrous that access to those does not need its own permission !
It's hard to see how you would implement that, any script run within the context of the page needs access to these fields for backwards compatibility reasons, so the context script of the extension would just need to find a way of running code in the context of the page to exfiltrate the data. It could do this by adding script tags, etc.
Browsers break backwards compatibility for security all the time. Most recently Chrome made accessing devices on a local network require a permission. They completely changed the behavior of cookies. They break loads of things for cross origin isolation.