Here’s a convenient little page that lists all the WebGL extensions that your browser supports, along with hyperlinks to their specification documents:

https://prideout.net/recipes/ExtensionViewer.html

Firefox 4 returns an empty list when calling getSupportedExtensions, but here are the results for Chrome and WebKit:

• Chrome 11 on Windows
• Chrome 11 on Ubuntu
• WebKit Nightly on Snow Leopard

Enjoy!