The Firefly 8S suffered an overheating problem while in use and on charge.This resulted in the battery expanding, and popping the front off the case.
As the camera has a rear screen the battery is access via the bottom side, and its expansion meant that it could not easily be removed. The Chinese website that I purchased the camera from were reluctant to offer a replacement (which is part of the risk in purchasing from this kind of site). They sent me a replacement battery instead, so I needed to fully dismantle the camera to get the battery out. The camera now works fine, although the front fascia won’t properly clip back on as the plastic grips snapped when the front was forced off by the expanding battery.
The processors in action cameras do appear to run quite hot, but possibly the higher resolution (i.e. 4K) the hotter they run. Other users have reported particular issues with the Firefly 8S running hot.