Excerpt:
First, install flashplugin-nonfree from the repos:
sudo apt-get install flashplugin-nonfree
Second, wrap the flashplugin-nonfree with nspluginwrapper:
nspluginwrapper -i /usr/lib/flashplugin-nonfree/libflashplayer.so
Third, create a link from the new file located in your home directory to the Firefox plugins directory:
sudo ln -s ~/.mozilla/plugins/npwrapper.libflashplayer.so /usr/lib/firefox/plugins/libflashplayer.so