Chrome has a lot of the frontend written in JS nowadays too. At this point XPCOM is mostly just a bindings layer between JS and C++ (that's what COM was suppose

Chrome has a lot of the frontend written in JS nowadays too. At this point XPCOM is mostly just a bindings layer between JS and C++ (that's what COM was supposed to be to begin with—a glue layer between languages). V8 has something similar. I don't think you can really say static linking is more effective in Chrome or Firefox. Both browser architectures are broadly similar these days.