try this
First, close running Firefox.
Open the terminal (shell prompt)
Install Flash Player
Type the following command to install :
$ sudo apt-get install flashplugin-nonfree
Some people might find voice is not working with newly installed flash player.
Type following commands to solve this problem
$ sudo apt-get install alsa-oss
$ gksudo gedit /etc/firefox/firefoxrc
Find line that read as FIREFOX_DSP and set to :
FIREFOX_DSP="aoss"
Save and close the file.
Close Firefox and restart it again.
>> Stay informed about: Flash plugin installation for Firefox browser