User Agent: Mozilla/5.0 (Windows NT 6.1; WOW64; rv:14.0) Gecko/20100101 Firefox/14.0 Build ID: 20120624012213 Steps to reproduce: JSON.stringify(window.performance.timing) => "{}"; also Object.keys(window.performance.timing) results in [], although using a for ... in loop enumerates all keys of that object properly. Actual results: "{}" Expected results: I expected a JSON string that properly serialized all properties of the window.performance.timing Object.

This is the behavior the spec requires right now, though there is talk of changing that. In particuar, JSON.stringify only looks at an object's own properties, while for...in looks at the prototype. And per spec all the relevant properties live on the prototype.