I'm not aware of any webgl apps in the marketplace, but my favorite demo has always been crystal skull. You can go to webgl.com to find the webgl-demo-glass-skull and view it in the browser. It almost seems to perform better if you then bookmark it to the home screen and run it again that way.
When tapping on the icon on Firefox OS 1.0.1, like with other packaged apps, I see a folder with files. To start the game I tap on index.html. I assume that this is a known bug with version 1.0.1. Anyhow, in the folder I see a file three.js, a library which wraps WebGL and other technologies for displaying 3D graphics in the browser. This indicates that the game could make use of WebGL. The game runs fine on my Keon!