How much of original code is left in modern software products? [closed]

Question

Many of you out there work for large companies that ship well-known software. I was wondering, how much of original code (basically, code that was "v1.0" release) is left in modern massive applications, such as, say, Firefox, Photoshop, Windows, Linux, etc? I'd really prefer first-hand experience and real-world war stories.

Thanks for satisfying my curiosity.

EDIT

Turns out there's a degree of misunderstanding. What I'm after is basically as follows: when you blame/annotate source code, are there any parts or even whole files untouched since the initial 1.0 release.