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.
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.