Pasco PD and Kennewick PD have been issuing weekly warnings not to, but it's actually ILLEGAL to warm up your car unattended -- that's true in all of Washington state.
People in Richland (16yrs and older) can use fireworks from June 28th through July 5th. Anything that flies could get you a $250 fine.
In West Richland you can use fireworks between the 2nd and 4th of July with restricted hours.
In Pasco and Kennewick fireworks are banned...
According to the Tri-city Herald :
Residents in Franklin County -- including Pasco -- and those in Kennewick and Prosser are banned from having fireworks of any kind.
Richland, Benton City and unincorporated Benton County residents, however, can use what officials consider "safe and sane" fireworks...