Don't send credentials on Firefox

Reddit doesn't send Access-Control-Allow-Credentials headers (at least not on GET requests; I'm guessing it doesn't on POST requests either), set xhr.withCredentials to false so that it doesn't error out.

Note: I'm not entirely sure whether Reddit sends it on POST requests so it's possible comments may be able to be posted; for now, this fix sets both GET and POST requests to not use credentials, but this may need changing.