nsIProperties Interface

 Gets a property with a given name. 

 @return NS_ERROR_FAILURE if a property with that name doesn't exist.
 @return NS_ERROR_NO_INTERFACE if the found property fails to QI to the 
 given iid.

void get(in string prop, in nsIIDRef iid, [retval, iid_is(iid)] out nsQIResult result)

  Returns an array of the keys.

void getKeys(out PRUint32 count, [array, retval, size_is(count)] out string keys)

 Returns true if the property with the given name exists.

boolean has(in string prop)

 Sets a property with a given name to a given value. 

void set(in string prop, in nsISupports value)

 Undefines a property.
 @return NS_ERROR_FAILURE if a property with that name doesn't
 already exist.

void undefine(in string prop)