Most people don't know this, but the original trio were going to be Spider Man, Iceman and the Human Torch (from Fantastic Four) this would explain why Doctor Doom, a classic Fantastic Four villain, appears in the original intro of the show.
But the studios were not able to get the rights to use any of the Fantastic Four characters, only Spider Man and the X-Men (and Iceman along with them), so they had to make up a brand new character that had fire themed powers. And so Firestar was born. They made her a mutant and join the X-Men (which they had the rights for already), and the fact that she was completely original allowed them to have a lot of creative freedom with her character and background history.
What I looked up for Firestar is somewhat depressing, she has potential among the Marvel heroes and heroines, but they just left her like that ever since. I also felt the pain of unrequited love several times, which made this relatable to me. Thank you for sharing this piece.
It depends on what you're looking at. In the comics, Firestar was dating/engaged to Justice, and as a former New Warrior she would join the Avengers at some point... complaining that Wasp's design for her costume was too revealling.
However, Firestar also appeared in the cartoon series "Spider-Man and His Amazing Friends" which consisted of Spider-Man, Firestar, and Iceman. In the cartoon, Firestar was an ex-X-man along with Iceman and had teamed up with Spider-Man after Firestar and Iceman learned Spider-Man's secret identity.
In the ultimate comics, Firestar is a different character from the regular Marvel U in the revalation that Firestar was Liz Allen and that she was the daughter of the Blob. However, after her becoming "Firestar" she largely dissappears from the series. I don't know if Marvel put her in the Ultimate X-Men line before Ultimatum in any serious way, but she did dissappear from the Ultimate Spider-Man title.