The new horns in 1.19 must have a new animation, as they cause a crash with the first person mod. Specifically, it calls this error: applyTransform() must not be called on a vanilla ArmPose!

Tested it with just this mod, with the latest version of the mod and Forge 43.1.1. Attaching the crash report.

crash-2022-11-06_01.01.54-client.txt

If I remember correctly, that was a forge bug, so update your Forge install. Not a FirstPersonMod issue.