There are two questions: Does such firmware exist? And which precautions does Linux provide?
The rules I formulated in that post you refer to were meant a bit broader, not just firmware, but all kinds of blobs, which could also include binary only drivers that are loaded as modules into the kernel or binary only libraries which can live somewhere between kernel space and user space. All that should IMHO not be acceptable since these can compromise the system.
Concerning precautions by the kernel or system, well, that’s not so easy since in general the kernel and Linux community wants to support such use cases - although of course no one really wants it, but it should at least be possible. So the kernel does not prevent this in any way. There are precautions like IOMMU and things that can help a bit but as soon as a driver runs code in kernel space as loadable module it has a lot of privileges.
Strictly speaking about firmware and the Linux loadable firmware interface I am fairly sure that it does not directly support loading kernel space executable code but I am also quite sure that it could be cleverly exploited to do so, if someone would write a malicious driver to do that. But then you have already malicious code (the malicious driver) in kernel space and then loading that firmware piece is just the icing on the cake.
Cheers
nicole