And, depending on how many floors up they are, they MIGHT survive, whereas they WILL NOT survive if they stay in the room.
Jumping out of a burning building might be desperation (to avoid a burning death, a reflex of self-preservation, etc.), but not necessarily despair.
It all has to do with the intention of the person -- do they intend to kill themselves? Or do they just intend to TRY to escape the fire, even if it might not work out? It sounds like death, in this case, would be an UN-intended consequence.
Now if someone jumped out of a building because the stock market crashed and their retirement account went from $200,000 to $1,200 -- now THAT would be ѕυιcιdє, and certainly a mortal sin.
I agree with Telesphorus. We can't dictate God's will. God's will is God's will, not ours!