In aircraft construction, what is the difference between a firewall and a bulkhead?
A bulkhead is any kind of structural panel separating two spaces in a fuselage.  Like a solid panel in the tail cone that has only small holes for cables to pass, or the panel that forms the end of the baggage area.  Pretty much any solid lateral partition with a structural use. On a large aircraft, any structural partitions separating parts of the cabin, like the flight deck bulkhead, or terminating the pressure hull at each end, the "pressure bulkheads".

A firewall is a firebreak.  It's a bulkhead that is fireproof to separate a "hot" area like an engine compartment from adjacent structure, so that if you have an engine fire, you'll have more time to try to land, or if at high altitude, say your prayers.  To be a firewall, the material of the bulkhead has to be flame resistant, with a much higher melting point than aluminum. 

On GA airplanes, the firewall is usually stainless steel, but can be galvanized steel (antiques and homebuilts), or even special flame proof coatings on a normal bulkhead (some composite homebuilts).  On transport airplanes, a firewall will be stainless or titanium.

So in a nutshell, a bulkhead is a structural lateral panel in a fuselage.  A firewall is a "fireproof" bulkhead normally separating the engine compartment (or other flame source) from the rest of the plane.