I was on a USFS helicopter rappel crew a number of years ago, before that, an engine leader, and volunteer for a rural fire district. I believe knife selection should be task driven. Thinking back, for me, that primarily consisted of:
-Opening boxes and packaging
-Cutting parachute cord
-Field sharpening tools if a file wasn't available
-Field programming Bendix King radios (you need to short a couple of contacts with a knife blade to enable)
-Food related (MRE pouches, prep)
I can't remember anyone who didn't have a knife, especially a multi-tool. In working with different crews, there was a fairly small number of blade savvy guys and gals. I recall seeing a whole lot of lower-end Bucks, Schrades, Gerbers and Leathermans. I knew more than one Hotshot who packed a CRKT K.I.S.S. Inexpensive and easily replaced if lost, lightweight, lanyard hole, and the grind allowed it to be sharpened with a flat file; which were usually around for sharpening chainsaws and other tools. I think it's important to recognize that most wildland firefighters are seasonal, with a significant number of them college students. Price point is a big deal. I carried a Rescue Jr. in my flightsuit, a Leatherman Supertool on my belt, and a dummy-corded PE Endura in my radio harness or line gear. Definitely not high-end, but a lot of people thought I was nuts for having such "expensive" knives.
I didn't see too many fixed blades; the ones I recall were compact, ala Schrade Sharpfingers. I did have a 3.5" partially serrated fixed blade made up by a local knifemaker when I got on the rappel crew, thinking it would get some good use. But honestly, the folders were easier to carry and never left me wanting. Ruana Knives out of Montana makes a knife called the Smokejumper that's been around since the 1930s. I believe TOPS does a production version that's considerably cheaper than the custom. It might give you some design feature ideas; for what it's worth, all of the jumpers I knew carried folders.
If I was doing fire again, at the very minimum I'd have a Gerber MP600 or Leatherman Wave+ multi-tool. For a conventional blade, I'd pair a sheepsfoot SE folder with a PE folder, both from the Salt line. I never had an issue with rust on the conventional steel blades, but like the characteristics of the Salt steels. And the loss prevention yellow and green handles are a big plus. In my opinion, FRN just works when it comes to hard-use Spydercos; especially when you factor in price, weight, performance, and availability. For the seasonal firefighter on a budget, the orange Cara Cara 2 Rescue makes a lot of sense; a good potential Fire Byrd.
Much respect to the current generation of wildland firefighters. The extreme fire behavior occurring in more and more fires these days was not very common in my time. Stay safe!