Bug 1186491 introduces a notification-based API that can be used to watch at almost no cost for jank. We can use this API to introduce a page jank telemetry. In a first version, this would only measure the jank of webpages (in a single histogram, without any information on the webpage itself). This will allow us to correlate the information with other information available on Telemetry, such as OS, CPU, e10s, hardware, etc. Future versions could extend this to discriminate based on the features used by the page (e.g. WebGL, WebASM, WebAudio, ...)

about:performance is being redesigned; mass closing the bugs related to parts of the current about:performance page that we are not keeping. Our goals with the redesign are to reduce the overhead caused by having the page opened, increase the reliability of the displayed information, and make the offered information actionable for most users. The back-end work is being tracked in bug 1419681.