Not necessarily a suggestion, but I have used Elm for it :-). The point being, use what you'd use anyway for a website. That said - with an extension some of y

Not necessarily a suggestion, but I have used Elm for it :-). The point being, use what you'd use anyway for a website. That said - with an extension some of your code is about interacting with existing pages. For that, much of the time vanilla JS might be good for compatability, since the page itself is already using X or Y or Z framework. For the menus, extension page and so on, or complicated UI you want to show on top of a page, you could use React.