The graphics really make that bike. How did I miss seeing this build?
Like a Hawthorne, Firestone is a brand name, the bike was built by Snyder, who also made Rollfast. The graphics may be unique to Firestone though. Your chain wheel is Murray, probably changed out years ago.
So if it's a 58, the serial number may be on the bottom bracket or rear dropout. Probably the bottom bracket as my list shows the transition to the dropout in 1959. You should see a 5 digit number with an 85 SN (or EH) as the year is reversed on Snyder bikes of that era. 75 would be a 1957 bike. I rode the same bike, although stripped down, back when I was a busboy for Denny's in the early 70's.
Great job on keeping the original paint and graphics!