Flash -- you either love it, or want to see it burning eternally in the fires of Hell. Mozilla's Shumway project, an attempt to create a replacement Flash plug-in that uses HTML5, might ever so slightly placate those barracking for the latter. Previously Shumway has only been available as a separate extension, but it recently made its way into Firefox's nightly builds, hinting at the prospect of mainline inclusion somewhere down the line.
As this story on TechCrunch explains, you'll need to do more than just grab the latest Firefox nightly and you'll still need Flash installed. However, once you do have the nightly, all you have to do is visit
about:config within Firefox and turn it on.
Shumway is still very much a work in progress and there's no guarantee when it will, if ever, replace Adobe's Flash plug-in. In fact, the project's introductory Wiki page on Github describes Shumway as a "HTML5 technology experiment" that might be integrated with Firefox "if the experiment proves successful".
If you do end up giving it a try, you can test it using the provided demos.