A few years ago, all the browsers depended on Adobe to release the latest updated version of Adobe Flash Player to fix some vulnerability that became known publicly. This meant that users of that browsers stayed exposed to Flash vulnerabilities until Adobe released the update. This is why many web browsers have started to create their own Flash versions or Flash alternatives. For example, Chrome and Opera use their own version of Flash, Microsoft Internet Explorer also comes with its own version of Flash and now Mozilla Firefox is also developing Shumway which replaces Flash for playing SWF movies and animations.
Shumway uses HTML5 for all the animation and movie playback and supports almost all the features that you would find in Adobe Flash player. Although Shumway is still under development, but you can still install it and use it in your Firefox browser. Shumway comes as an extension file that you can download and install from its github web page http://mozilla.github.io/shumway/.
You can see the Shumway extension installed under the Add-ons web page in Firefox that you can open by typing about:addons in the Firefox location bar. There are no options for the Shumway extension, so there is nothing you can configure. But you can disable the extension if you want to use Adobe Flash Player instead of Shumway.
When you try to visit a web page containing SWF files, the Shumway extension loads all the SWF movies or animations. You can see the Shumway logo on the top-right corner of the SWF movie. Compared to Adobe Flash Player, Shumway is very slow but consumes less of your system resources.
Conclusion: Shumway is Adobe Flash Player replacement for Mozilla Firefox. It comes as an extension for Firefox and can be easily installed and used in a matter of seconds.