The power to "set someone on fire" can be as simple as just being a part of the Fire descriptor. A GM ruling for a setting could go something like this:
Fire Descriptor: Most people hit by a fire attack have something flammable on their person and, until they take a standard action to extinguish it, are considered a source of fire for the purposes of Pyrokinesis and other power that require existing fire. People who choose to remain on fire take Continuous Damage 1 (same as putting your hand on a torch and not pulling back immediately).
This will make it easy for "super-tough" characters to shrug off low-level "on fire" effects no problem via Impervious, but normal human characters may have some issues with being on fire if they don't have even a single rank of Impervious Toughness. Other ways to deal with this are having a free descriptor for a "super-suit" that makes it "Fire Resistant". Aka, it's not the characters in the comics who are so fire resistant, but the suits.
The idea of that descriptor ruling is not to punish people who aren't immune to fire effects but to acknowledge that even though a flamethrower doesn't have Continuous damage, yes, it sets people on fire. Because that's what it does. And there's an infinite number of ways to say "No, you don't set me on fire" or "I'm on fire, but all you've done is give me an awesome special effect".
Higher-level fire attacks, such as covering someone with burning super-napalm, should probably be statted as Continuous Damage, with the Ranged or Perception as is appropriate to the rest of their descriptors, not to mention Fades if there isn't more heat being continuously added. Because countering powers is a thing, the effect's continuous effects can be countered by anyone capable of "stop, drop, and roll" unless it has some other descriptor like "Fires of Hell". High-level pyrokinetics might also elect to not have Fades to represent their ability to continue stoking the flames while fighting other things.
I dislike Secondary Effect for modeling this effect unless it has a lower rank (about half) than the original effect. If a fire is able to continue burning at full strength for six seconds with no additional fuel, why does it suddenly extinguish itself in the next six seconds? Again, some types of fire in the comics do not obey the rules of fire, but I can't cover every brand of fire in the various multiverses.