A fire inside the fuselage is the worst possible emergency one can face. You have very little idea of the exact location or severity of the fire, but you know the consequences of it.
Jumping the checklist or rushing is usually not a good idea, but with an onboard fire the checklist is rarely useful beyond the obvious, and direct cockpit-crew firefighting attempts are ill-advised for a number of reasons.
All you can do is to get on the ground ASAP and in order to do that you have to be able to see: the instruments - the ground - something - anything. A sucessful autoland with smoke in the cockpit due to a fire somewhere inside the fuselage is a simulator fantasy.
Anthing you have to do in order to see should be done - forget the book... the clock is ticking; you're going to die.
Opening a DV window (if you have one) depressurized, inflight doesn't cause reverse airflow from the cabin into the cockpit. Very little actual flow occurs due to the fuselage becoming "pressurized" by the forward motion of the airplane, unless you open something in the cabin - such as an overwing exit - which becomes extremely difficult due to the pressure you've created. Once you do manage to get the overwing exit open, there's quite a bit of flow (front to back) and it gets noisy - but you can see. (personal experience, below 5,000 ft and 250 kts).
Someone mentioned the onboard fire that killed Ricky Nelson - here are the firsthand observations of the duty copilot on that flight:
On-Board Fires