You can always remove certificates from CAs you don't trust. If you don't trust Verisign, how can you tell Amazon is really Amazon?
Moreover, even as a sophisticated user, I have to observe that I actually do not trust a single root certificate in my browser the requisite 100% that is necessary for this whole system to work. I can not assign a 0% probability to any of them being compromised by a hostile force, especially when the hostile force isn't necessary a random hacker but may be a sovereign government.
So, while your point is totally correct, it should be pointed out it isn't really a defense of the current system. (Which you may not have intended it as, but I have seen others use it this way and it's worth pointing out.)
Right now, I won't trust it that much.
Further, there's no room for decreasing my trust in a cert based on the length of the chain, or who's on the chain, or anything like that.
The whole system is structured to produce a 100% certification of trust, or a 0% certification of trust. This has the consequence that when the system is compromised in the way that you talk about in your last paragraph, it doesn't degrade; it falls apart.
It's not that robots from the future might compromise a cert, and going to something so fantastic in the light of the real threat of sovereign governments and the proved threat of compromised root CA certs (proved as in "you could buy this device for real money and have it really delivered to you") is a rhetorical unkindness. It is that nobody can any longer claim with a straight face that the system is 100%, and unfortunately, with SSL that leaves only one alternative.
Compare with the semi-mythical "web of trust", which would have ways of dealing with this that doesn't violate the very mathematical foundations of the system.
The implication, and what makes this scary, is that if this were done with malevolent intentions, there could be a whole host of sites out there that had "bogus" certificates that Firefox would think were fine because they were signed by this trusted authority.
Just as an aside, you can always see the certificate chain in Firefox by clicking on the favicon area when a certificate is in use (i.e. when using https), clicking on "More Information", clicking "View Certificate", and going to the "Details" tab. You can then see the "Certificate Hierarchy". This shows you all the certificates in the chain, and who signed what. At the very top should be the "root authority"; this is usually someone like Verisign.
To bring this whole thing around, then, on some sites, you could see the root authority as the certificate mentioned in the post.
I guess, finally, I should point out, like others have, that this doesn't really mean anything bad is going on; the certificate could be perfectly valid. However, it is at least a little worrisome that they aren't sure where it came from.