Mozilla started to work on Shumway, an open source Flash environment that did not depend on proprietary software back in 2012.
It made sense for Mozilla to work on Shumway at that time, considering that Flash was still the dominant force on the Web, and that Adobe did not cooperate with Mozilla in regards to the integration of Flash in Firefox like it did with Google.
Mozilla's goal was to integrate Shumway in Firefox in a way that it would allow Firefox users to access Flash content on the Internet without having to rely on Adobe Flash much like the company added a native PDF reader to Firefox to eliminate the need for plugin-based PDF readers.
Development of the Flash replacement slowed down considerable in September 2015 and rumors began to spread that Mozilla changed its priorities in regards to Shumway.
Shumway on the Firefox Graveyard
If you check out the main Shumway tracking bug for implementation of the feature in Firefox, you will notice that it has been moved to Firefox Graveyard which means that work on it has more or less stopped.
Mozilla engineer Chris Peterson revealed in a comment that Shumway is no longer a priority for Mozilla.
Shumway is still on GitHub, but it's not on a path to ship in Firefox. Given our limited resources, other product priorities are currently more important than propping up Flash.
Flash lost much of its dominance on the Internet thanks to the rise of HTML5 and web-based technologies that deliver content to Internet users without the reliance on proprietary technology.
Media streaming is without doubt the area where Flash lost most ground but Google's announced that it will not accept Flash banners on its ad network from January 2, 2017 on, and that will impact the use of Flash significantly as well.
Considering that Mozilla had plans to enable Shumway for Flash advertisement first, as listed under Milestone 3 of the project, it may have influenced Mozilla in the decision making process as well.
In addition, Mozilla plans to end support for NPAPI plugins in Firefox for 2016 which means that the organization would have to have Shumway ready by that point in time to provide users of the browser with a replacement, or extend support for Flash instead.
It is possible -- theoretically at least -- that Mozilla will restart development on Shumway in the future but the likelihood of that happening is slim considering that Flash is being replaced by new technologies on the Internet. (via Sören Hentzschel)