Install the Flash plug-in

Flash is a plug-in for your web browser that allows you to watch videos and use interactive web pages on some websites. While HTML5, which is a more modern solution, is increasingly offered, there are still some websites which won't work without Flash.

If you do not have Flash installed, you will probably see a message telling you so when you visit a website that needs it. Flash is available as a free (but not open-source) download for most web browsers.

Adobe has announced that they will stop updating and distributing the Flash Player at the end of 2020. Consequently, for security reasons, the Flash plug-in should not be used after 2020.

There are two kinds of plug-ins available:

NPAPI for Firefox and other Gecko-based web browsers
PPAPI for Chromium and other Blink-based web browsers including Opera and Vivaldi

The Google Chrome browser is shipped with Flash bundled, and does not need a plug-in.

flashplugin-installer

The flashplugin-installer package provides the NPAPI plug-in for Firefox. Install the package and restart your browser.

adobe-flashplugin

The adobe-flashplugin package provides both the NPAPI and PPAPI plug-ins, i.e. it provides Flash for both Firefox and Chromium like web browsers.

Make sure that the Canonical Partner repository is activated.
Install the adobe-flashplugin package.
Restart your browser.

browser-plugin-freshplayer-pepperflash

Some features of the PPAPI plug-in, such as 3D acceleration and premium video DRM, are not available in the NPAPI plugin. If you use Firefox or some other Gecko-based browser and need those features, you can install the browser-plugin-freshplayer-pepperflash package. It's a wrapper which makes Firefox use the PPAPI plug-in.

Make sure that adobe-flashplugin is installed.
Open a terminal window by pressing Ctrl+Alt+T and install the browser-plugin-freshplayer-pepperflash package:

sudo apt install browser-plugin-freshplayer-pepperflash
Restart your browser.

Web browser provided as a snap package

If you use Firefox or Chromium as a snap, the packages described above do not help. Instead the steps below are suggested to make Flash available.

Please note that you are supposed to always use the latest version of Flash. It means that you need to repeat step 2 - 7 once in a while to keep accessing web services which require Flash.

Firefox as snap

Chromium as snap

Create a lib folder:

mkdir ~/snap/chromium/current/.local/lib
Go to https://get.adobe.com/flashplayer/.
Download the .tar.gz file for Linux.
Navigate to the folder for downloaded files.
Extract the files from the downloaded archive file:

tar xf flash_player*
Copy libpepflashplayer.so to the lib folder:

cp libpepflashplayer.so ~/snap/chromium/current/.local/lib
Restart your browser.

More Information

Web browsers — Change the default browser, install Flash…

https://help.ubuntu.com/stable/ubuntu-help/net-install-flash.html.en