Large parts of the Firefox browser (as distinct from the Gecko+SpiderMonkey) are written in JS. Have a look at the code some time. Or, just open chrome://browser/content/browser.xul in Firefox to get a taste.
The use of XUL and the resulting UI clunkiness (speed, responsiveness, nativeness) are pretty well known.
A world in which only the vendor gets to write low-level code sounds is a terrible division of labor.