1. Go to Adobe’s Flash Player download page.
2. Open a Terminal window
3. In the Terminal window, change to the directory where you saved the file you downloaded (e.g. cd /home/user/Downloads)
4. Extract libflashplayer.so from the file you downloaded with the command tar -zxvf
5. As the super user, copy the extracted file, libflashplayer.so, to your Firefox installation directory’s plugins sub-directory. For example, if Firefox is installed in /usr/lib/mozilla, use the command sudo cp libflashplayer.so /usr/lib/mozilla/plugins and then enter your super user password when prompted.

source

Tested in debian 9.x