It occurred to me that merely having a Flamethrower, Promethean Crawlers and Flood Stalkers would be enough. No need for specific spider enemies. As long as the Flamethrower is useful, fun to use, unlike the one in Halo 3 (seriously, the slow movement doesn't bother with other "support weapons" but with the flamer... it kills all the fun from that weapon).
I think the decision to make the Flamethrower a support weapon rather than having a first-person incendiary firearm was deliberate. The fact that incendiary damage is one of the only sustained-damage effects that not only affects shielding and health, but that the player can aim with a consistent level of accuracy, means that significant time had to be poured into it to make sure it was not only fun to use, but
safe enough to be employed in encounters. Firebombs were dropped by Stalkers and had a few occasions where you got to use them against Covenant, but those also have a much smaller radius and a potentially shorter amount of fire - not to mention, as with all grenades, you're throwing an area-of-effect weapon
away from you, rather than using the attack at a close range like the Flamethrower. Most instances where you got the Flamethrower were against large amounts of Flood, so the support weapon implementation became extremely useful compared to treating it like a run-and-gun weapon. The third-person perspective gives you some extra visual indication that Flood are approaching from multiple directions and notes potential verticality in a way the motion tracker doesn't, it allows for tighter (slower) aiming and as a result more precise burns, and the slower movement speed forces defensive, backpedaling play - a necessity when the flames wouldn't outright kill certain Flood forms in the first few seconds, meaning you have to be coordinated with the Flamethrower or risk having a mass of still-burning infected charging you in CQC.
Halo PC had a first-person flamethrower with wonkier physics, and there would be instances of "invisible flames" depending on what settings your particles were at. As a result, people would end up walking into their little patches of fire a lot more than you'd expect.