OK, so the actual difference on the original site is from a disagreement about
top: 100% on the relatively-positioned element with class ".dropdown-menu". Chrome's right, and this is our bug, bug 1092007.
Chrome is disregarding that style (i.e. resolving the style to 'auto' which in the case of relpos means no-adjustment).
Firefox is honoring that style (resolving it against the height of its container).
Chrome is correct; the container in this case is a flex item whose size should be considered indefinite, because it's in an indefinite-size flex container and has an indefinite (content-based) resolved flex-basis value.
Here's a reduced testcase demonstrating the difference:
Chrome renders "Side by side" all on one line, whereas Firefox shifts "side" downwards (due to mistakenly honoring its percentage value on its
Status: NEW → RESOLVED
Closed: 2 years ago
Resolution: --- → DUPLICATE