This feature has been removed from the Web standards. Though some browsers may still support it, it is in the process of being dropped. Avoid using it and update existing code if possible; see the compatibility table at the bottom of this page to guide your decision. Be aware that this feature may cease to work at any time.
js-ctypes allows application and extension code to call back and forth to native code written in C. C++ support is possible through
vtable pointers see Using COM from js-ctypes. For a discussion on extended C++ support see bug 505907. Other work made possible by js-ctypes is JNI, this is elaborated on in the JNI.jsm section and not the js-ctypes section due to the JSM abstracting away all of the js-ctypes. Unlike binary XPCOM components, it allows developers to ship a single binary for use with multiple versions of Firefox.