CssRules No Longer Works In FireFox 1.5

I have a WYSIWYG editor based on the HTMLArea project that uses the
cssRules object of a stylesheet to add/update stylesheet rules for
table cells. Ever since I upgraded to FireFox 1.5, I get the following
error message when trying to access the cssRules object:

"Access to restricted URI denied"

The code I am trying to execute looks like this:

var currStyleSheet = editor._doc.styleSheets[0];
var currRules = currStyleSheet.cssRules;
var tdIndex = currRules.length;
for(i = 0; i < currRules.length; i++) {
if(currStyleSheet.cssRules.item(i).type == 1) {
//if it is a style rule
var currRule = currRules.item(i)
if (currRule.selectorText.toLowerCase() == "td") {
tdIndex = i;
}
}

}

This code is simply looking for any rules for TD elements. I am at a
loss as to what is going on and/or what changed if FireFox to cause
this error. It worked fine in previous releases of FireFox. The
document that I am trying to access the stylesheet for is within an
IFRAME, which is what I believe is causing the problem.