This guide assumes you have just installed Burp, opened it once, and immediately closed it because the screen was too busy. That is a reasonable response. Here is the path back.
Step one: the proxy
Open Burp. Go to Proxy → Settings. There is exactly one row: 127.0.0.1:8080. Leave it alone.
Step two: Firefox
Browsers default to talking directly. We need them to talk to Burp first. In Firefox: Settings → search 'proxy' → Manual proxy → 127.0.0.1, port 8080.
Step three: the cert
bash
fetch the cert
curl --proxy http://127.0.0.1:8080 \http://burp/cert -o burp.der
Import that into Firefox: Settings → 'certificates' → Authorities → Import.