Most people at this forum have a bad habit of constantly imagining how a battle would go in a comic and not how it is supposed to go if the characters were created in real life. Of course in a comic this would be no cakewalk for Superman (but he would still win in a comic against both).
Assume that a Genie where to create these characters from a wish based off their majority showings in order to develop an accurate power set for the characters. The Genie would gather all comic evidence for both Firelord's and Terrax's durability against physical punches or strikes and it will see that there exists no evidence whatsoever that these beings can even take a punch from Superman without dying. Even though Superman has some low showings, the Genie would only take the majority of consistent showings to develop his power set as well. This means that Superman would be both fast and strong enough to kill both Firelord and Terrax in less than 3 sec.
This thread is spite to me in favor of Superman and people are actually arguing that the team will win when they can't win at least once in an infinite amount of matches. I guess lack of evidence for someone's durability and battle speed (reflexes) don't count anymore. I guess it is valid just to say that someone can do anything without proof. I guess people are emotionally debating things in which they are trying to feel for the truth instead of look for it.
Last thing, PIS is defined as either one doing something below and against what is shown statistically throughout their career or it is defined as one performing a feat that is an outlier-a rare feat that is not established statistically throughout a character's career. Firelord and Terrax has never shown the level of physical durability to survive a punch from Superman and in which they have shown a level in which Superman would kill them in one punch. So them not being able to survive a punch from Superman is not PIS since statistically they don't have the physical durability nor reflexes and speed to do so.