Step 1 : Get the libflashplayer.so file from Adobe offical website

Step 2 : Extract the libflashplayer.so file to a folder

Step 3 : Go tp the folder which sores the libflashplayer.so file

Step 4 : Run the following command in terminal
             sudo ln -s libflashplayer.so /etc/alternatives/libflashplayer.so

Step 5 : Start the Firefox and now can watch flash