Weren't there safety standards for AC-DC radios (Underwriters Labs, et al.) when this Firestone set was new?
If not, IMHO, there should have been, as some of these older AC-DC sets (especially those in metal cabinets, such as Arvin's model 540T, et al.) could pose a real hazard if the line plug was inserted in the socket a certain way (i. e. so the chassis is hot). I would think your Firestone radio would have been designed from the start with isolation networks, etc. so that there is nothing in the radio to create a shock hazard.
I hate to say it, but my Zenith H511 5-tube set from 1951 doesn't seem to have any kind of isolation network between the line cord and the chassis either. When were radio manufacturers required to design safety features into AC-DC sets? I can't believe this type of radio was in production so long (since the '40s if not earlier) without having protection against electric shock (except, perhaps, for plastic knobs and/or cabinets). I cannot begin to imagine how many people were shocked, injured or killed by these poorly-designed radios (from a safety standpoint), although some of these accidents could have been avoided had the radio been used in dry areas and not, for example, in a bathroom. I read in an old appliance-repair book some years ago of an incident involving an AC-DC radio that was being used in a bathroom, while the owner was taking a shower. He reached out to turn up the volume or tune in another station, the radio fell off the shelf it was sitting on, and fell into the water; the fuse protecting the bathroom circuit immediately blew. I read of another incident (in the same appliance repair book) in which a young mother was giving her baby a bath in the kitchen sink, with a plugged-in electric frying pan nearby. The child started waving its arms around wildly, got caught in the cord, and pulled the appliance into the water. Needless to say, the baby was killed instantly.