Back in the days Adobe Flash used to be very popular technology. Although, Adobe plans to end support for it by 2020, there are still a lot of web sites relying on this outdated technology. In October 2017 the new version of the popular GNU/Linux distribution Ubuntu with codename artful was released. This is the first version that comes with GNOME shell on Wayland.
After updating my Ubuntu 17.04 to 17.10 I had issues with Adobe Flash player in Mozilla Firefox 56.0 (64-bit). I had to executed the following commands in a terminal to solve the issue:
sudo apt install browser-plugin-freshplayer-pepperflash pepperflashplugin-nonfree
sudo update-pepperflashplugin-nonfree --install
Execute the following command to verify the installation and check flash version:
sudo update-pepperflashplugin-nonfree --status
For example, here is the status on my machine after the successful installation:
leon@leon-ThinkPad-T460s:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description: Ubuntu 17.10
leon@leon-ThinkPad-T460s:~$ sudo update-pepperflashplugin-nonfree --status
Flash Player version installed on this system : 126.96.36.199
Flash Player version available on upstream site: 188.8.131.52
Please note that you must restart the web browser before the new installation of Adobe Flash will take effect.