----- Original Message ----
> From: Noppanit Charassinvichai

> Dear All,
> I want to develop an extension that somehow it has to write a string to
> a xml file. How can I do that? because I have to save some kind of
> options that user chooses.

See http://developer.mozilla.org/en/docs...trees_to_files
for complete example

> Do I have to use XPCOM?

You have to use built-in XPCOM components. See http://developer.mozilla.org/en/docs...trees_to_files
You don't have to write your own XPCOM component.