There will be no pressure differential between upper deck and main deck, following activation of the Main Deck Fire arm and depress switch (otherwise it would be impossible for crew member(s) to descend to the main deck to check/fight the cause of the warning). As to the "comfort", No.1 pack will probably struggle to maintain a comfortable temperature in the cockpit and upper deck area (depending on the O.A.T at 25,000ft). The temperature on the main deck will probably decrease towards O.A.T. (unless fire suppression was unsuccessful!).
It is a "plan to land at nearest suitable airfield" checklist (ditching may be necessary), which is why there is a lack of time information.
- the above refers to the 747-400F.