Non-standard
This feature is non-standard and is not on a standards track. Do not use it on production sites facing the Web: it will not work for every user. There may also be large incompatibilities between implementations and the behavior may change in the future.
This Mozilla-specific method is available only from within privileged code, and is only called from a C++ context in order to initialize an XMLHttpRequest. To initialize a request from JavaScript code, use the standard open() method instead.