I have a local SSE server that is sending messages on port 8080. I would like to read them from the browser using EventSource as shown here.

But if I type the following in the browser console:

var evtSource = new EventSource('http://127.0.0.1:8080', {});

I get an error:

Firefox can’t establish a connection to the server at http://127.0.0.1:8080/.
Content Security Policy: The page’s settings blocked the loading of a resource at http://127.0.0.1:8080/ (“connect-src”).

How can I fix this?