One of my visitors pointed me to the fact, that it's currently a bit difficult to find old Flash Player 6 versions for testing purposes as there is only v7 served. You can get all old version from v5 down to v2 from the "Flash Player Museum" at the Macromedia site. But what's currently missing on that page is version 6. I searched the Macromedia site and finally found one location where you get it: The "More Downloads" page - you will have to download the whole Player 6 Update package though. And I recommend you to use the Flash Player Uninstaller first.

I already added a request to add version 6 to the museum page in the "How can this document be improved box", but if you do, too perhaps it will happen faster.