What you need to play Shockwave content under GNU/Linux with Firefox and Media Player Classic is WINE (http://www.winehq.com/), the Windows installer version of Firefox (http://www.mozilla.com/en-US/firefox/all.html) and Shockwave Player (http://www.adobe.com/downloads/)
After you've installed WINE, install, in order, the Windows version of Firefox and Shockwave Player
Once you've installed all of them, set Media Player Connectivity like in the image below:
What's important is the highlighted entry. In the first field put the path to the WINE executable, for example /usr/bin/wine
and in the second put the path to the Firefox executable installed under WINE, followed by the “%f” argument.
Assuming that you haven't changed any of the default options, it should be:/home/your_username/.wine/drive_c/Program\ Files/Mozilla\ Firefox\firefox.exe %f
You're done!
Many grateful thanks to Underpass from Mozilla Italia and Sethnakht for help and suggestions