Step 1 :
apt update
apt install browser-plugin-freshplayer-pepperflash pepperflashplugin-nonfree
Step 2 :
Go to https://github.com/cybernova/fireflashupdate to download fireflashupdate.sh
wget https://raw.githubusercontent.com/cybernova/fireflashupdate/master/fireflashupdate.sh
chmod +x fireflashupdate.sh
./fireflashupdate.sh
Step 3 :
To update Flash Player :
./fireflashupdate.sh
Step 4 :
You may need to change the settings at Preferences menu of Firefox Add-ons - Plugins - Shockwave Flash from "Ask to Activate" to "Always Activate".
That's all! See you.