They're mostly bitter anti-child people who rail against what they see as entitlements that parents get that non-parents don't. They derisively call parents petty and mean things like "breeders" and seem to be a very cynical bunch. I'm not saying their feelings are always ridiculous; certainly some of them have reasonable reasons for feeling the way they do. But they're a mostly-toxic, vocal minority.
It really annoys me when people assume all (or even a significant number) of childfree people are like those reddit folks (not accusing you of that, just saying in general.
And I don't get the automatic association between FIRE and childfree that some people are making here. Sure, FIRE is easier if you don't have kids, but IME the two groups are only loosely connected, at most.