Permission to obtain SVGDocument.getElementById property denied

Hello I am trying to make an application where I can control an SVG image from another browser window. The communication between windows is ok, and my receiver gets the info, but when I try to access to the SVG file to change something firebug tells me that I don't have permission to obtain SVGDocument.getElementById property :S

To simplify, my SVG file is embeded like this:

<embed id="car1" height="100" width="500" pluginspace="http://www.adobe.com/svg/viewer/install/" type="image/svg+xml" src="car1_5.svg" name="car1"/>

And the way I test the access to it:

window.onload = function()
{
svgdoc = document.car1.getSVGDocument();
var door = svgdoc.getElementById("door1");
door.setAttribute("stroke-opacity",0.4);
}

What is strange is that it lets me get the SVG document... but then I can't access to the "door", wich is an element of the SVG.

Ah... and I don't have any problem with IE... it works fine there :S Can anybody help me??? pleaseeeeee:)

Thank you in advance