Flashvars stopped working in Firefox 3.5.2; work in FF 3.0, IE, and Safari

Has anyone else encountered this issue and/or know of a way around it? I have a swf that pulls data from Flashvars. In IE and Safari, it works just fine. In Firefox 3.0, it did and does still work (I had a co-worker verify this). However, in the latest version of FF, I get this error:

TypeError: Error #2007: Parameter url must be non-null.
at flash.net::URLStream/load()
at flash.net::URLLoader/load()
at index_fla::MainTimeline/frame1()

The flashvars line in the webpage is:

<PARAM NAME="FlashVars" VALUE="xmlFilename=<?php echo $xmlfile; ?>&fontVar=<?php
if ($CSS == "mainplusone.css") {
echo "15";
} elseif ($CSS == "main.css") {
echo "13";
} ?>" />

If I change the line to use direct variables instead of php code, I get the same results - script errors in Firefox, no problems in Safari or IE.

The actionscript to read this line is:

var myURLRequest:URLRequest = new URLRequest(loaderInfo.parameters.xmlFilename);
var fs = parseInt(loaderInfo.parameters.fontVar);

If anyone can suggest a way around this, it would be greatly appreciated... If I need to post additional code, let me know. I can't post the URL as it's an intranet.