Sailor Fire didn't have the same upbringing as the other Sailor Senshi. She was the first to meet Sailor Dark and was granted dark powers, which gave birth to Dark Fire. As Dark Fire, she could fly and shot black fire balls at her enemies, but Sailor Dark had control over her. There was a little voice (the real Sailor Fire) that she ignored until it went onto Sailor Light. While in Sailor Light's mind, Fire told Light what happened in order to avoid asking for help.

Once she was healed, she was questioned by Sailor Light about what happened. Sailor Fire remembers what happened from before she met Sailor Dark. As it turns out, she was jealous of Sailor Light's looks and the Negaverse started feeding her small amounts of dark energy*. Sailor Fire is cautious about how she feels now and keeps herself in check.

*There's a small contradiction here. In chapter 16, it's explained that energy has to be accepted. Well, that's only true for large amounts of energy. You can get very small amounts of energy without accepting.