Any ideas why it is hit and miss?
Do you test as much on FF as on Chromium based browsers, during development, as well as in quality assurance steps? I could imagine, that some devs are looking at it in Chromium (or even Chrome shudder) all day, while implementing things, and then just having to tick a checkbox for FF "Yep, seems to work.". Which would of course lead to a Chromium-centric code.
Most devs here use FF as their daily driver - that said, if you need to debug an emscripten-compiled c++ WASM application in the browser, then it's either printf-debugging, or Chrome + the C/C++ DevTools extension. Additionally, the overall QoI for running WASM binaries appears quite a bit better for Chrome. Both issues tip the scales currently, in favour of Chrome (sadly).