Obsolete
This feature is obsolete. Although it may still work in some browsers, its use is discouraged since it could be removed at any time. Try to avoid using it.
DOMUserData refers to application data. In JavaScript, it maps directly to Object. It is returned or used as an argument by Node.setUserData(), Node.getUserData(), used as the third argument to handle() on UserDataHandler, and is used or returned by various DOMConfiguration methods.
Note that although it can be an object, in Mozilla, it may be returned as a string or other type, if it was set as such a type (e.g., Node.setUserData() and Node.getUserData()).
DOMUserData is not persisted nor serialized and will not be present after the application has restarted or after a crash. If you are interested in persisting data you might rather need to use nsISessionStore.