“Permission denied to access property 'href' from a non-chrome context” in JavaScript and OpenLayers

问题

This is really two questions one leading into the other. Firstly what does 'Permission denied to access property 'href' from a non-chrome context' actually mean?

Secondly I am using overlays in OpenLayers and wish to change the opacity of said layers, this is where this error is being thrown the code of which is here...

<input id='opacity' value="1.0" onChange="changeOpacity()">

Of which changeOpacity() is the following function...

function changeOpacity() {
var newOpacity = parseFloat(OpenLayers.Util.getElement('opacity').value);
newOpacity = Math.min(1.0, Math.max(0.1, newOpacity));
OpenLayers.Util.getElement('opacity').value = newOpacity;
for (var i=0; i<images.length; i++) {
layers[images[i]].setOpacity(newOpacity);
}
}

which throws the error at "var href = originalElement.href;" here...

function mD(e) {
//what is originalElement/srcElement/originalTarget?

var originalElement = e.srcElement || e.originalTarget;

 var href = originalElement.href;

  if (originalElement.nodeName == "A" && href.match("http://www.openstreetmap.org/browse/")) {

    href = href.replace('http://www.openstreetmap.org/browse/','http://www.openstreetmap.org/api/0.6/');

if (gml) { map.removeLayer(gml); } //$("status").innerHTML = 'loading'; }

gml = new OpenLayers.Layer.GML("OSM", href, {format: OpenLayers.Format.OSM});
map.addLayer(gml);
gml.preFeatureInsert = style_osm_feature; 
var sf = new OpenLayers.Control.SelectFeature(gml, {'onSelect': on_feature_hover});
map.addControl(sf);
sf.activate();
return false;

   }
}

Any help/ideas is great appreciated! I am using firefox 3.5.9 and firebug 1.5.4