This feature alone has made me switch to Chrome. Without PIN support for Yubikey, there really is no point in supporting WebAuthn at all, in my opinion.
Hard agree. If you've turned on Two-Factor Security with Keys for your iCloud account, every time you try to use "Log in with Apple," you're challenged for your security key. If it's PIN protected—which it should be—your login fails, so "Log in with Apple" is rendered useless.
Put it another way, without this feature, it rules out Firefox as a capable tool if pin based Yubikey is a requirement for an environment (eg corporate or other). So there will be waves of replacements of Firefox with alternative capable browsers as organizations align with best practices around MFA (especially since a swathe of recent hacks using MFA fatigue pushes organizations to choose stronger security measures like hardware tokens) and it will add to the rhetoric that Firefox just isn't a secure option vs competitors due to lack of security features, slow uptake of best practices, as more users see these features as necessary rather than nice-to-have.