My take would be that if you have the engine fire light on, then half the bleed ducting may well be full of smoke and fumes. If you continue the checklist it has you turning the left pack on and potentially putting all that dodgy air through the pack and into the cabin. The only time I can think of when it would be a problem with a number two engine fire is if you have needed WAI earlier and selected the isolation valve to Auto and opened the isolation valve thus filling the entire bleed ducting with contaminated air. Standing by to be corrected by those much smarter than me.