Yea, Firestone no longer makes the Firehawk in 275/60-15's.
There are a few manufacturers that do though. Gotta dig deep to find 'em, though. Hankook, Cooper, BFG, Mastercraft (?), Nitto, Kumho, Dayton, etc., etc. But most are 14-20 year old technology in these sizes. If the shipping wasn't so stupid, I'd consider Euro spec tires. They seem to have a greater choice when it comes to 15" tires.
The truck'll be needing new shoes in the spring, & I've already begun searching.
Been considering these...
'Federal Couragia S/U's'.
They're an 'H' rated tire. About $100.00/ea. Good looking tire! UTQC of 400, so they are fairly hard, which should make 'em last a while, but certainly not comparable as far as stickiness goes, to a tire with a 300 UTQC. 300 & lower UTQC rating tires will not have a long wear period.
Had some 'H' rated Bridgestone tires on my El Camino, which were replaced with some T/A's. The 'S' rated T/A's were crap when compared to the B'stones. Corners that I could go into @110k on the B's & accelerate through, I wouldn't even think to attempt @ 90k on the T/A's.
I've also been looking at 'Hankook Ventus H101's', 'S' rated. Only because of the size is right, & they kinda look cool! No idea on how they perform or last. Probably similar to the T/A's.
Then there's these. Kumho Ecsta STX's. Speed rating of 'V'! Overkill for sure on a truck! Decent price for what they are.