OpenSnitch is a comprehensive open source Linux application firewall

This is not a simple firewall to get started with as it can be a bit of an uphill battle to get all the outgoing connections vetted (if you want to filter those), but I'd imagine once set up, it is a pretty powerful tool to have.

Key features:

* Interactive outbound connections filtering.

* Block ads, trackers or malware domains system-wide.

* Ability to configure system firewall from the GUI (nftables).

* Configure input policy, allow inbound services, etc.

* Manage multiple nodes from a centralized GUI.

* SIEM integration

It is regularly updated, with 74 contributors contributing to over 30 releases. The project has also received over 10,000 starts.