I’m not sure about the statute of limitations. If they spent 4 years still in California and then moved out of state for 6 years will they still be charged if they confess in this situation?
The statute of limitations has almost certainly run out. I believe it would be one year on the fireworks offense itself and three years on the false statements to the police. There are some exceptions to the general rules for statute of limitations, but I can promise you that nobody cares about a 10-year-old fireworks case.