I have followed all the instructions in the wiki page, I made:
sudo apt install libflashplayer-nonfree
But after the installation the folder
/usr/lib/flashplugin-nonfree doesn't contain the library
libflashplayer.so, there is only the
pubkey.asc file. So Firefox can't see the plugin. I also have tried to find that library in other locations but it does not seem to be anywhere.