Shumway is still very experimental and not working for all SWF content, but it's moving along and evolving rapidly. Shumway should be especially interesting for mobile devices where Flash Player support tends to be lacking. The Shumway implementation comes down to five components: Core, AVM1, AVM2, Browser Integration, and Testing/Demos.
Details on Mozilla Shumway were announced in this Mozilla Research blog post. The code is currently being hosted on GitHub where it's described as, "Shumway is an HTML5 technology experiment that explores building a faithful and efficient renderer for the SWF file format without native code assistance. Shumway is community-driven and supported by Mozilla. Our goal is to create a general-purpose, web standards-based platform for parsing and rendering SWFs. Integration with Firefox is a possibility if the experiment proves successful."
This non-native implementation is what differentiates Shumway from the other open-source SWF / Flash Player projects like Gnash and Lightspark.