about:telemetry: diff mode + search

I cleaned up about:telemetry to be a bit less of a hack. It mostly generates charts using proper DOM manipulations now, so should be easier to contribute to.

Diff Mode

Now that we have a few hundred histograms in telemetry, it’s become a chore to figure out if something changed. I added a ‘Diff’ button which re-polls telemetry since last telemetry-page-load/diff-press and highlights buckets with new activity in red. This is useful for cases when you see lag in the browser, but have no idea what’s causing it (sort of like about:jank). This turns histogram bars red when stuff changed. Unfortunately one still needs a pretty intimate knowledge of the browser to figure out if any of the histograms are related to observed lag.


I added a little input box to make finding relevant histograms quicker.

Help Wanted

If you find any bugs or have improvements to contribute, see the source. The updated addon is in the AMO review queue, in the meantime you can jump the queue via this link.


  1. Since hangmonitor.timeout provides telemetry data in nightlies instead of crashing the browser, is this something that will show up in about:telemetry?

  2. Is this the about:jank page?. Or is this completely different.

  3. see subject, it’s about:telemetry

  4. Mardeg: The data from the main thread hang reporter will be exposed in the next about:telemetry version.

    The stacks produced by the hang reporter aren’t useful to users until we publicly deploy a symbolication Web service.