Are you sure Firefox *doesn't* use IE infrastructure? They've introduced a very similar patch today, and I see no reason why they'd wouldn't do exactly the same thing Opera does w.r.t. XML.

Ditto Google Chrome, which also released a patch today. It certainly *could* be coincidence, but until I see something telling me that Firefox hasn't done exactly what Opera did, I'm suspecting that as the most likely explanation.

Relying on the OS for one's core functions isn't exactly unusual.