1. Download the latest adobe flash player installer from
https://get.adobe.com/flashplayer/
2. Launch "Terminal"
3. Extract the directory from the compressed folder (which looks something like "install_flash_player_11_linux.x86_64.tar.gz" for 64-bit) by typing in the Terminal
tar -xzvf install_flash_player_11_linux.x86_64.tar.gz
4. Move to the extracted directory, type
5. Install, type:
sudo apt-get install flashplugin-installer
6. Launch Firefox and check if you have Adobe Flash Player installed (
http://www.adobe.com/software/flash/about/)