Is it possible to create custom XUL elements from XPCOM or NPAPI?

I was wondering if it is possible to create a new XUL component via any available api, such as XPCOM or NPAPI, so we can use it our XUL files.

Let's say I wanted to clone the XULs vbox's components code and add a few modifications to it, so we could use our custom XUL component just like this:

<window>
    <myvbox mycustomarg1="customValue"> Some content... </myvbox>
</window>

I know what XBL is and what is used for and it doesn't fit our need.

Any suggestion of how to achieve that?

Edit:

We need to create a browser component in Firefox as child of another browser object. The problem is some websites detect this child browser as iframe and we want to avoid this.

Thanks.

Solution

If the point is preventing a webpage loaded into a frame from messing with your XUL document then you should use <browser type="content"> - this establishes a security boundary between chrome and content which (among other things) prevents the content document from accessing its parent frame. It is important however that your XUL document itself is loaded as chrome and not content (by either being on top level or inside <browser type="chrome">). See https://developer.mozilla.org/en/XUL/Attribute/browser.type for documentation.