As of 16.04, Fresh Player is available in the official repositories. Installing is as simple as:
sudo apt-get install browser-plugin-freshplayer-pepperflash
However, if you still want the most up-to-date version, you can add the PPA:
sudo add-apt-repository ppa:nilarimogard/webupd8 sudo apt-get update sudo apt-get install browser-plugin-freshplayer-pepperflash
This will install the actual player, but it still needs an extra file. This file is the actual
pepperflash library used by the player. To get it, you can use a PPA:
sudo add-apt-repository ppa:skunk/pepper-flash sudo apt-get update sudo apt-get install pepflashplugin-installer
Close your browser and re-open, and it should be enabled. The reason Youtube and Vimeo worked most likely was because of the support of HTML5 video on those websites.