The movie will be written for Jerry Bruckheimer Films and Walt Disney Pictures; Disney optioned the book last year for $1.5 million before it was even published by Random House, Daily Variety reported.
Firth has been on the screenwriting scene for more than a decade. He surfaced in 1994 for his series, "All Quiet on the Preston Front," and later was known for "Neville's Island" and "Border Café," among others. Firth's two feature films were "Calendar Girls" and "Blackball" in 2003.
The story by the son of the late rock icon Frank Zappa is about a brother and sister who learn their family's business is monster hunting.
It is only a matter of time before they must come into their own, fighting "the most diabolical creature in the universe," Daily Variety reported.