This is a no-go for me as I have numerous websites secured with Yubikey and without this the browser is just "YT/Facebook viewer"...
As soon as this changes, I might give it another chance.
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EDIT: Oh, FFS I just tried to look it up again and found property "security.webauth.u2f" which seems to be false by default. After enabling it U2F works! Geez, why would you disable that id default -_-