Shumway has now landed in the latest Firefox nightly builds ahead of Firefox 27. Mozilla isn't enabling Shumway by default right now but it's just a matter of setting it up through the about:config page. Shumway right now can handle many Flash applications and such, but not many commercial applications at this time.
It's good to see Mozilla Shumway moving along swiftly when many other open-source Flash projects are moving slowly and not seeing new releases.
Confirmation of Shumway landing in Firefox -- along with additional information -- can be found via this blog post.