Looking good George! As far as the fire wall is concerned, here are my thoughts on it if I may.
I would think that, even if well maintained (kept clean) there would still be some discoloration to the areas from oils and lubricants staining the paint. In my experience as a crew chief on Blackhawk helicopters, we would try to keep our aircraft as clean as possible, inside and out. We didn't leave oil or grease around the engine areas or the hydraulic deck, and frequently washed away dirt and grime. From a maintenance perspective, it is much easier to tell if a seal is going out and trace where a leak is coming from if the aircraft is kept clean. We also touched up chipped paint by hand when found to prevent corrosion, as well as applying anticorrosion films to exposed unpainted hardware and sliding/moving components such as hinges. We were land based, not near any saltwater, but still kept up with anticorrosion measures constantly. Now, that being said, it has been my experience that no matter how well we cleaned our aircraft, some paint discoloration from POL fluids and even the anti corrosion products was evident. Where the anti corrosion film was applied, the areas would appear slightly glossy and the color darker as it was a somewhat oily compound and left the area appearing wet. The areas I am talking about are not wide swaths or patches, but just around rivets and bolt heads since the compounds were applied to just these spots with an acid brush, not much bigger than we might use in our hobby (1/2 inch wide at the most). Where airframe skin overlapped and components bolted/riveted to brackets etc., you could see some grime just under the edges at times that was just near impossible to get too, but not filthy by any means.
Ok, sorry for the long ramble, but if I were going about this on my model, I would at a minimum do some pin washes around the rivets and airframe stuffiners(?) On the firewall. I found a picture of an FM-2 wheel bay/firewall that illustrates this nicely I think.
Dunno if this is static display or airworthy, but still it can be seen that the firewall is pretty clean, but also a bit stained.
Just wanted to offer some personal insight in case it may be of help. Either way you decide, she is still coming up very nicely and I am enjoying your build very much.
-Chad
Pic via the internet image search