Once the extension is installed, you can check the WebCL support by clicking here. Firefox displays a warning message before displaying the WebCL support:
Now that WebCL is okay, you can play with Kernel Toy, a kind of WebCL live coding tool:
I slightly modified the code to extract the luminance. I have two OpenCL platforms on my system (one from NVIDIA and one from AMD) and by default, Kernel Toy uses the first OpenCL platform. If someone knows the trick to use another platform, that would be cool.