Firehole sizing is just out of whack.
I like their hooks, think they're good quality, nice finish, and hold up well...but I will never order them online unless it's simply to replace old stock, simply because the sizes they list are nonsense numbers.
Now I know sizing is subjective from brand to brand, but at the end of the day, "size" has always been determined as a measure of gape. So now that hook manufacturers are doing weird shit like "wide gap fly hooks", sizing numbers mean less now even than the little they used to.
I tie blobs on a bunch of different hooks, and while the Firehole 637 is top notch, I'm tying on 8s and 10s from everyone else...and the Firehole 12 is bigger than a 6 of most brands. Not even close.
So yeah, sizing is a crap shoot, but Firehole is bad at it, even by current industry standards.
On that note, I'd really, really love to see us get away from "sizes" and do it all with a simple 2-value system in millimeters, gap-shank. So a 6.5-12 would be a 6.5mm gap, 12mm shank, for an approx. #10 3XL nymph/streamer hook equivalent. We could still have the normal "heavy/fine wire", and eye descriptors, but that'd really help out the sizing quagmire we have today.