Nice config, mobile phones don't require an AV and even if you somehow get attacked through a sophisticated zero day exploit it will be out of the AV's scope to detect it because the bad guys mostly attack existing apps on your device to break into your device and gain root access (which may trip Samsung's KNOX counter on your device but I'm not 100% sure about this).
Only malicious .apk files can be detected by Android AVs but that shouldn't be a problem as long as you have good internet habits and some common sense.
Best advice for anyone reading would be to get a phone that receives security patches constantly however Google started to send security updates for critical Android components independently from the OEM with Android 10.
Latest Firefox (codename Fenix) blocks external SSL certificates so any attempt of MITM (like Adguard system-wide adblocking) won't work, maybe you can try excluding Firefox from Adguard settings and install uBo or Adguard extension on Firefox.