[ Unfortunately, it doesn't take the existence of firebenders for Liv to assume the use of fire against someone like her- just a basic understanding of how old horror movies go. Zuko is just the first person she can consider the possibility not involving some kind of propellent. ]

Something someone else mentioned. It's like.. regular humans with differences in their genetic- [ Wait, he probably won't- ] in how they were born that makes them present with varied physical attributes. So a mutation would be like if one person's eyes were two different colors or, apparently, superpowers in some worlds. Sometimes the mutations are neutral or beneficial to the species and they become more prominent, whereas others end up being a hindrance to survival and you end up with genetic disorders. There are a lot more technical details about mutations, but those are the CliffNotes. If firebending is common in your world, it's probably just a common beneficial mutation.