If you tried to install it through firefox but it just never work, you might want to consider downloading it from adobe.com directly.
1. go to adobe.com. Click on the get flash button.
2. Download it and “unzip” it.
3. Use terminal to go the directory and type sudo ./flashplayer-installer. The installer will ask you where you install your firefox. Usually it’s installed in /usr/lib/firefox. Then just simply restart firefox and it should work fine.