It's asking for a Firefox plugin that will install OK, but doesn't work with Windows Media Player 12.
It works with Media Player 11 and before.
If you have the "IE tab" add-on installed in Firefox you should be able to switch over to Internet Explorer 8 and play this file.
Internet Explorer has native support for playing a file like this in Media Center 12.
And no, I don't think downgrading Media Center 12 to 11 is an option.
I've run into this issue before and a Google search turned up the problem I outlined above with no solution.