> The JS code won't be able to modify the browser itself, but neither can ELisp modify the actual Emacs binary code.

You misunderstood the problem. Remember when Firefox' GUI was made of XUL and that was indeed accessible from JavaScript (before they abandoned add-ons in favor of Google's extensions). Then you had add-ons like Firebug or KeySnail that could indeed take control of the browser's "chrome" (i.e. the visual elements that don't belong to the HTML page, s.a. tabs, menus, address bar, buttons around it).

If you don't remember... life was a lot better back then. I've built a tool for interactively highlighting selected DOM elements in the page from a dedicated toolbar for example (this was before the developer tools). You could completely remap all browser keys. You could call external editor to edit the text in text areas or text inputs. You could call programs on the local filesystem (there used to be an FTP add-on that did that).