Installing Plugins (Flash, Shockwave, etc.)

Can someone update the information in above mentioned section?

Since ages you can't choose in the Shockwave installer which browser you want to install it in, because it bluntly installs itself within Windows.

Installing Flash Player via the XPI doesn't seem to work anymore either. It used to before but now it's complaining about a missing install script or something like that (error 204).
It might be something missing in a previous install, I don't know for sure at the moment.