The point is that You shouldn't set preferences to just download stuff because you don't actually know what you are downloading.
The code from the presentation is available here (It is Java but I'm sure you can easily get the gist of it):
If you are determined to just download anything and not check what has been downloaded the relevant part for Firefox is in:
You would need to do this for each driver you support, it is a per browser configuration.
The question still remains, what are you trying to test? That a browser can download files? What are you going to do if it can't? Can you fix the browser and force your users to use your modified version of the browser?