2. Executar:
sudo add-apt-repository ppa:nilarimogard/webupd8; sudo apt-get update; sudo apt-get install freshplayerplugin
3. Reiniciar o Firefox;
4. Testar: adobe.com/br/software/flash/about
4.1. Caso ocorra o erro "Failed to load libpepflashplayer.so", instalar o Google Chrome [download AQUI].
Ou, executar os seguintes comandos:
sudo apt-get install pepperflashplugin-nonfree
sudo update-pepperflashplugin-nonfree --install
Sobre o Fresh Player Plugin: linuxuserlog.blogspot.com.br/sobre-o-fresh-player-plugin-pepper