In case the automatic installation of Firefox plugin fails, you can use the following Manual steps:
Go to http://get.adobe.com/flashplayer/otherversions/
Choose Linux 32-bit / 64 bit depending on your OS
Choose latest Flash Player on Step 2 and download the file
Once download is complete, right click on the flie (tar.gz) and click on “Extract here”
Launch Terminal (Keyboard Shortcut : Ctrl+Alt+T)
Go to that location where you extracted the file
Run this command : sudo mv libflashplayer.so /usr/lib/firefox-addons/plugins
Restart Firefox..
Enabling Flash on Firefox – Ubuntu