We begin by downloading Fennosys ROOT CA:
Do not attempt to install it from Firefox (It'll refuse to do it). Instead we now go to..
* Android Settings → Security & Lock Screen → Encryption & credentials → Install a certificate
Select “CA Certificate”
Select “Install anyway”. The device will now ask your PIN. Enter the PIN and continue.
Tap on “Fennosys-PN-CA.crt”
So our CA is now installed to Android's CA store!
Next we'll setup Firefox to use Android CA store so that it'll work with the CA we just added.
All done!