We begin by downloading Fennosys ROOT CA:

• https://manuals.c1.fi/certs/Fennosys-PN-CA.crt

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.

• Open Firefox → Settings → About Firefox
• Click Firefox logo 5 times (There will be small pop-up saying “Debug menu is enabled”)
• Back to Settings
• Secret Settings → Activate “Use third party CA certificates”

All done!