I am able to solve this by:
1. Copying the certs from /home/jakob/.config/Local/run/router/nginx/certs/ to /etc/ssl/certs/
2. Running sudo update-ca-certificates
2. Manually adding the site's cert as a Certificate Authority in Firefox (Preferences > View Certificates...> Authorities > Import...)
I am on Linux Mint 18, but should help with MacOS as ssl chain is similar.