I'm trying to view a webpage which requires Flash. I'm using the latest Firefox Quantum on Ubuntu 16.04. However, on Firefox, the part of the webpage that requires Flash is just greyed out. There's no pop-up bar or an option to enable Flash. I went to about:plugins to see if Shockwave Flash is disabled, but I only found 2 plugins there - "OpenH264 Video Codec provided by Cisco Systems, Inc." and "Widevine Content Decryption Module provided by Google Inc.". There's no sign of any Flash. I can view the webpage fine on other browsers, like Chrome and Vivaldi. While the flash content is blocked by default, it says "click to enable Adobe Flash Player", which is followed by a pop-up box that asks for permission to enable flash, and after that the content plays just fine. How do I enable/install Flash on Firefox?