Fire
Explanation
The Joke
A woman runs up to firefighters screaming "My house is on fire!" The firefighters initially seem helpful, but one of them starts nerding out with irrelevant scientific tangents instead of actually fighting the fire. He explains that in zero gravity, there are no convection currents, so the fire would form a sphere, and it would need to "draw in more oxygen" differently, and could potentially "extinguish itself rapidly." He continues with more physics trivia about how fire in orbit would behave differently.
The exasperated woman asks him to actually help, but he reveals: "I'm a theoretical firefighter." This is the punchline -- he's the firefighting equivalent of a theoretical physicist, someone who studies fire academically rather than actually putting it out.
The Humor
The comic satirizes the stereotype of the academic or theoretical scientist who is fascinated by the abstract properties of a phenomenon but completely useless when it comes to practical application. The juxtaposition of a life-threatening emergency (a house fire) with a calm, lecture-style explanation of combustion physics creates the comedic tension. The term "theoretical firefighter" is a brilliant absurdist invention -- it parodies the distinction between theoretical and applied/experimental fields in science, applying it to a profession where the theoretical/practical divide would be genuinely dangerous.
References
The zero-gravity fire facts mentioned in the comic are actually real science. NASA has conducted experiments showing that flames in microgravity do form spherical shapes rather than the teardrop shapes we see on Earth, because without gravity there are no convection currents to draw the flame upward. These experiments have been conducted on the International Space Station and in drop towers. The comic accurately describes this phenomenon while using it for comedic effect.