it should be noted that the "id" attribute on elements inside actions
(or generally inside templates) is generated by the template. what
happens exactly is described here
http://www.mozilla.org/docs/xul/xulnotes/template-primer.html

If that page is correct, multiple elements may get the same id.
anyways, document.getElementById() does generally not return the
visible onscreen element and some internal bindings (like docShell
- designMode ?) also seem to break inside a template.

Yes, getElementById would return the element inside the template. You should avoid using ids inside a template - Neil