I've had to debug around a short-lived bug in the Firefox rendering engine that only showed up in the FF rendering engine before. The site itself? Standards-compliant; there was flexibility in the standard (ironically, for flexboxes ;) ) about performance constraints and our site just fell down Gecko's slowness tree and hit every branch on the way.

Given the engines are allowed to vary, this experience is inevitable from time to time. The user-agent is really valuable in helping us figure out when that's what we're seeing.