I figured out the problem.
FireFox doesn't support the 'LoadXML' function. See http://www.w3schools.com/Dom/dom_parser.asp for the details.
the JavaScript in /Controls/SolpartMenu/spmenu.js needs to be changed as follows in the SolpartMenu.prototype.GenerateMenuHTML function.
OLD:
if (this.xml.length)
oXML.loadXML();
FIXED:
if (this.xml.length)
try //Internet Explorer
{
oXML.loadXML(this.xml);
}
catch(e)
{
try //Firefox, Mozilla, Opera, etc.
{
var parser=new DOMParser();
this._m_oDOM = oXML=parser.parseFromString(this.xml,"text/xml");
}
catch(e) {}
}