Is there any free NPAPI plugin that could provide full access to local file system?
Maybe there are some ready-for-use universal plugins which provide access to many OS native api functions?
Such a plugin would be highly dangerous; Generally people are bright enough not to leave things like that just laying around.
You could build such a thing using FireBreath, but it is very difficult to lock it down enough that nobody could create a malicious website that would use it to trash your computer.