var ccMan = Components.classes["@mozilla.org/charset-converter-manager;1"].createInstance(Components.interfaces.nsICharsetConverterManager);
var encoders = ccMan.getEncoderList();
var list = [];
while(encoders.hasMore())
{
    list[list.length] = encoders.getNext();
}
if (list.length > 1)
{
    // Здесь кидает exception
    var encoder = ccMan.getUnicodeEncoder(list[0]);
}