Follow these simple steps...
1. apt-get install flashplugin-nonfree-extrasound
2. create a symbolic link to let know firefox to use libflashsupport.so plugin
2.1 reach this folder : mozilla-firefox/plugins (usually its under /usr/lib/mozilla-firefox/plugins)
2.2 execute this command ln -s /usr/lib/flashplugin-nonfree-extrasound/libflashsupport.so
3. Restart firefox and you are done.
Just simple as that...
Please note: apt-get libflashsupport is not supported anymore.
