If both a strict XHTML doctype tag and an XML prolog are written into a popup using separate document.write commands, Mozilla/Firefox/Gecko based browsers will revert to quirks mode, instead of standards compliant mode.
Click here to open the popup (writes using separate document.write commands) - Firefox 1.0 final release fails
Click here to open the popup (writes using a single document.write command) - Firefox 1.0 final release works
JavaScript and requested popups must be enabled for this demonstration.
This has been reported to the Mozilla Organization as bug number 268442.