CSS Grid overlay doesn't show

Say when I view mozilla’s CSS Grid page, the dev tools won’t show the grid overlay. It does show the representative grid in the tools though. Also clicking the grid icon on the display: grid property, won’t work.

Any ideas?

What version of Firefox are you using?

Do you see any errors in the Browser Console?

Firefox 56.0 on macOS HS.
When I select an overlay grid in the layout view, I get a bunch of warnings:

Unexpected value NaN parsing x1 attribute.  markup.js:335:6
Unexpected value NaN parsing x2 attribute.  markup.js:335:6
Unexpected value NaN parsing y1 attribute.  markup.js:335:6
Unexpected value NaN parsing y2 attribute.  markup.js:335:6

Also now and then the toggle gets stuck, not being able to switch it off anymore.
Edit: the toggle acts like a radio button with the other toggles.

Where can I file a bug report? I installed FF Dev Edition and am seeing the same problem. For a brief moment fiddling with the Grid Display Settings toggles could see the overlay, but cannot repeat it to show again.
Edit: found bugzilla.

Yes, Bugzilla is the right place!

The Firefox :: DevTools: Inspector component is the best match I think. If you already filed, post a link here, and I’ll help guide it to the right people.

I haven’t been able to reproduce myself, so it’s great to file a bug like you are doing.

Ok, that would be great. Filled it at https://bugzilla.mozilla.org/show_bug.cgi?id=1412419