In the linked Codepen demo, hovering over the black "Social" box will cause it to flip down, revealing some red circles. Moving the mouse over those circles quickly in Firefox causes them to disappear, where they do not in Chrome or Safari.

I'm not sure what would cause this vanishing, but it seems like a noteworthy interop issue given that the demo isn't particularly complex to my eye.

REgression range : https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=e94a014eb27e172be7f6b3860806803159d3d1f2&tochange=99d782b09b7d036db2596201c7dae8baf06de85e

Suspect : bug 779598 (which makes sense as the CSS has preserve3d)

Dupe of : bug 1698988

The test case in codepen is good for testing (because the example in bug 1698988 is a real site and is 404 now).

And thanks to add the test to that bug. :)