The only thing that makes Fusion Fire and any of the books in the Firebird trilogy worth reading are their extremely original settings and surroundings. Otherwise, the plots and characters struggle for originality.
Fusion Fire continues Firebird in a predictable way. Firebird has become pregnant with her and Brennan's twins, and is on the run from the Neithans, her former family. But in their way is Dru Polar, a Three Zed mad scientist who experiments with the mind.
Dru Polar is a typical villain to go right along with Kathy's other cast of typical characters. He's too evil to be believable. Firebird and Brennan are too perfect to be believable.
When Firebird and Brennan got married, a forge occurred between them through which they can read each other's thoughts and feel each other's feelings. This is one of the more interesting plot points.
But then Kathy introduces an odd plot point: fusion fire. after Firebird turns (who knows what that means) during the inevitable birth of her twin boys, she and Brennan can create fusion fire when together. Fusion fire is apparently a powerful weapon that can destroy people's minds therefore kill them. But they only use it one in this book.
When they get inevitably captured by Dru Polar, Tyers uses fusion fire as a CRT (Convenient Rescue Technique). A CRT is an idea that the author comes up with to get the heroes out of the bind they're in. This is usually a fantastical idea or tool that the heroes found before (kind of like James Bond). See Literary Trash.
Anyway, since fusion fire came in handy in this situation, it comes off as convenient. But Kathy Tyers is a growing author and has much to learn.
But while the public will be wowed by this book, I am not.