WTF is a fire box?
 

1991 740 non-turbo 2.3L

I have oil leaking from the cam seal on the front of the engine. I had a mechanic tell me the the fire box located on the driver's side of the engine could be clogged and causing too much oil pressure and making it leak from the gasket.

What the flip is a fire box?
I could not find any reference to it in my manual or a parts catalog.
Does it go by another name or is this the equivalent to a left handed bacon stretcher?

Maybe he meant flame trap.

OK
Is this something that would cause an oil leak because it was clogged?
Is it something that can be removed and cleaned or does it need replaced?

the flame trap is on an oil seperator box. the whole mess gets totally clogged up if its neglected and not cleaned periodically, and once its plugged up, your oil seals tend to start failing.

the flametrap itself is easiest to just replace, the oil seperator box itself can be cleaned if its not too old and cracked, otherwise it should be replaced.

they are a pain to access, as they are under the intake manifold. IPD sells a kit that puts the flame trap on a riser, seems like a good idea, but I've never tried one.
Flame Trap Relocation Kit - LH

note, turbos don't have the flame trap, just the oil seperator box... instead of the flame trap, they feed the fumes into the intake plumbing before the turbo.

I don't believe the 740 I4 has the system like the 850's so I'm not sure they have an oil separator box.

240/740/940's have oil separators and they are the first place to look when oil leaks start to appear. All Red Blocks have them, it is their PCV system. The flame trap actually is a no brainer to clean now and then but few separators get serviced which often will plug them up resulting in high positive crank case pressure which in turn blows seals. A way to check is to place something flexible like a rubber glove on top of the oil filler neck with the car running; the pressure should be slightly negative.