Assim sempre instalei o Flash Player no Firefox:
- Ir aqui
- Selecione ".tar.gz para outro Linux"
- Clique em "Fazer o download agora"
- Abra o Terminal com Ctrl + Alt + T
- Executar
mkdir ~/.mozilla/plugins/
- Executar
mkdir ~/Downloads/extracted/
- Executar
tar xf ~/Downloads/install_flash_player_11_linux.x86_64.tar.gz -C ~/Downloads/extracted/
- Executar
mv ~/Downloads/extracted/libflashplayer.so ~/.mozilla/plugins/
- Executar
rm -rf ~/Downloads/extracted/
- Executar
exit
- Reinicie o Firefox