We all know Flash is on the way out, but like Windows XP, it's been astonishingly stubborn. Too many sites still use it, and it's a common fallback method for displaying multimedia in antiquated browsers. And the plug-ins for Flash are notoriously unstable.
The demo page has several examples, which require the extension in the current release build of Firefox to be useful. Among them are a simple racing game, a number of graphics demos, an MP3 player, and a falling-boxes physics-engine example.
If one of Mozilla's goals is to do away with all browser plug-ins forever, it has a friend for life in me.