Documentation is a bit sparse, but it's using the contextmenu attribute which is new in HTML5. It uses the menu HTML tag which is now different from its HTML4 implementation. Browser support isn't so great, but it is supported in Firefox 8.

Here is a screenshot of the DOM on the page showing the relevant HTML, the contextmenu attribute on the body and the menu tags which are expanded.

UPDATE: This answer previously stated that Chrome 36 and up supported this feature, as previously stated by the caniuse page. This appears not to be the case and caniuse has removed this information.