You don't need to write Firefox extensions in C/C++ to do raw IO. The same interfaces Firefox uses to implement sockets and streams are accessible by JavaScript. I wrote a web server in ~200 lines of code.

You also don't need to write C/C++ to call functions in system libraries in Firefox. js-ctypes (libffi under the hood) works quite well, although converting headers to js-ctypes function definitions can't be automated IIRC, which makes it a bit of a pain.