Why don't you just fiddle with the <pubDate>Mon, 03 Nov 2008 21:37:50 EST</pubDate> value until Firefox/IE is showing the correct date?  Try time zones other than EST (maybe it's expecting GMT, as would've been output by HTTP::Date), increment/decrement the hour, etc.  Shouldn't be too hard to eventually figure out how Firefox is interpreting the pubDate value...