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Because I'm not sure we are talking about a normal windows userland and installed apps with add-ons.

How in the world would, for example, No-Script or Symantec stop a disgruntled employee from stealing passwords or other information from some web service company?

And what is your definition of "third-party" here, because I may be assuming facts not in evidence. Third-party relative to the OS, or third-party relative to your chosen browser?

I may be (mis)interpreting your post in at least two ways, I suppose.