It's illegal because it would be aiding copyright infringement to provide links to the files. This is due to the fact of BIOS files being a copy of the System software of a console.
What I can say that if you can dump the files from your own hardware, that's perfectly fine. I can also say that typing sensible things into google and clicking on the right link will get you somewhere. The legality of downloading the files from the internet is debatable, since it obviously won't have been dumped from your own hardware (whether or not the BIOS dump is identical to everyone else's is irrelevant)