Lithium battery fire?
If you have a warning that a serious lithium battery fire is starting while cruising along, I suggest you might be likely to find out whether it's real or not rather sooner than you have time to land.
Ideally, the batteries should be packaged so that a chain reaction fire is unlikely or would develop gradually. It seems that wasn't the case onboard the UPS6 747 in Dubai.
If the batteries are uncharged, then the suggestion to take the cabin pressure above 20,000 feet to starve the fire might be worth a gamble. But unless you can flood a well-ventilated fire with water, I don't think there's much hope of putting it out.