And since it's become so prevalent and so many people are familiar with it, it's being used for far more than it was initially intended for.
A simple Flash racing game running via Shumway in Firefox
"Shumway is an HTML5 technology experiment that explores building a faithful and efficient renderer for the SWF file format without native code assistance," Mozilla describes the project.
"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," it adds.
Performance is pretty solid, considering the early stage, though it's still noticeably slower than using the Flash plugin even with the simple demos provided by the project.
It's an interesting project that could prove very useful, if the team can pull it off. Replacing the Flash player, which has always caused problems, with a native web alternative would be a major win for Firefox. But getting Shumway to the point where it can replace the Flash plugin is probably going to take a lot of time and resources.
In the end, it's unclear if this is even needed. There are several projects that work on converting Flash files into HTML5, i.e. something able to run natively without any performance penalty. Both Google and Adobe are working on it. And, of course, more and more developers choose HTML5 rather than Flash in the first place.