Earlier quoted context omitted.
Seen a few comments like this. Old Version of Android? Because my FF on Android 8.1 renders it flawlessly.
What fonts are present (and therefore what characters will render successfully) depends not just on the version of Android, but also on the decisions of the device vendor about what to include.
I have an Android One phone running 8.1.0. I would normally expect font issues to be related to the underlying OS, but then I don't understand why Chrome works and Firefox doesn't.