How to install Flash on Debian Stretch?

The flashplugin-nonfree package is no longer maintained, if you need the Flash plug-in you should install it manually:

1. Download the latest release of the plugin in tar.gz format from Adobe.

2. As root, extract the downloaded archive and copy libflashplayer.so to /usr/lib/flashplugin-nonfree.

3. Fix the file’s ownership and permissions:

   chmod 644 /usr/lib/flashplugin-nonfree/libflashplayer.so
   chown root:root /usr/lib/flashplugin-nonfree/libflashplayer.so
   

4. If necessary, install the alternative so Firefox will find the plug-in. If

   update-alternatives --list flash-mozilla.so
   

   returns /usr/lib/flashplugin-nonfree/libflashplayer.so, it’s set up correctly (this would be the case if you had the plug-in working in the past), but if it doesn’t, you need to run

   update-alternatives --quiet --install /usr/lib/mozilla/plugins/flash-mozilla.so flash-mozilla.so /usr/lib/flashplugin-nonfree/libflashplayer.so 50
   

For future upgrades, you only need to repeat the first three steps.

Alternatively, pepperflashplugin-nonfree still works and will install the Flash plug-in for Chromium. You’ll need to download the package manually and install it using dpkg -i, but it will download the plug-in and set everything up for you. You can keep the plug-in up-to-date by running update-pepperflashplugin-nonfree --install (and check its status using --status).