We wondered about this, having occasionally hearing the 'pop pop bang' of fireworks on New Years Eve.  Legal?

According to state law, yes. But, the fireworks laws in Benton and Franklin County are the SAME as those for the 4th of July.  Illegal in Kennewick, Pasco, and all of Franklin County.  Legal in Benton County and Richland, but only certain types. No aerials or huge mortar shells like what you see over the river.

There's actually a legal fireworks sale window, where they can be sold between December 27th and 31st, from the hours of noon to 11pm.  We don't recall seeing any fireworks stands popping up around here, largely because not THAT many people light them up, or if they do, they get them elsewhere.

State law allows the discharge of fireworks (where locally legally permitted) between the hours of 6pm New Years Eve and 1am New Year's Day.

But again, if they're not legal in the city or county where you live, it's a moot point. But if you do hear some pop pop bang bang, as long as the folks are shooting them off in a permissible area, they are legal-for a short time.