The Adobe Flash Player is still used in many websites, especially for games. These websites need to provide workarounds for their visitors to play the Flash content.
Please, dear Mozilla developers, continue development on Shumway.
Some (maybe) useful refs:
@Anonymous Ruffle does not support ActionScript 3. Also, I'm not talking about an extension, but the self-hosted solution.
I use a version of Flash that's still supported on a portable Firefox fork but, this seems good for newer browsers.