Opps, check that. Avira comes with a firewall. It also installed Opera. I’ve used that in the past but haven’t in recent memory.
I use Avira but have not gotten Opera from it or had my browser default changed. It might be because I’ve had Avira a long time and haven’t used a newer fresh install option.
If it is doing that, certainly either don’t use it or make sure to uninstall Opera.