Solved: OpenSnitch, https://github.com/evilsocket/opensnitch available via Software Manager on Mint as well.

There was a windows firewall (tinywall?) that worked like this:

1. Something wouldn´t work on the internet.
2. I’d query the firewall to see what was just blocked.
3. I’d whitelist that traffic.

Pretty painless and I never was forced to think about the firewall until absolutely necessry, and then only to the least possible amount.

Is there any firewall on Linux that works similarly? Running Mint.