M1 (Bug 1470379) and M2 (Bug 1478396) have been mostly implemented during the 64 nightly cycle. They're not quite ready for prime time yet, so we will make them ride the trains with 65 only to release. However, we think they're useful enough that we would like to get more user feedback now. So this bug is to enable them in DevEdition 64. Practically, this probably means doing this // Flexbox preferences // Enable the Flexbox highlighter and inspector panel in Nightly and DevEdition #if defined(NIGHTLY_BUILD) || defined(MOZ_DEV_EDITION) pref("devtools.inspector.flexboxHighlighter.enabled", true); pref("devtools.flexboxinspector.enabled", true); #else pref("devtools.inspector.flexboxHighlighter.enabled", false); pref("devtools.flexboxinspector.enabled", false); #endif in /devtools/client/preferences/devtools-client.js

Pushed by pbrosset@mozilla.com: https://hg.mozilla.org/integration/autoland/rev/ba7c1249b79b Enable both M1 and M2 milestones for the Flexbox inspector in DevEdidtion 64 (and keep it there). r=gl