The linked JSON page isn’t at the same origin and directory as the document.
Please fix this.
And apart from chrome, there are other browsers in the world.

It works great for the most part, but you have to put files in the right places, or it won't find them.

This add-on is a dumpster fire

Não carrega os ficheiros JSON se for a partir de um link externo, tenho de fazer o download e colocar a localização do ficheiro para que os testes funcionem.

Doesn't work.
@CodingCaptain: That's not the problem, it still doesn't work. I'm getting the impression Udacity is trying to get people to switch to Chrome and is consciously and deliberately ignoring the firefox problem.

Jeez! you get reviews from a about a year ago that it's not working properly with local files and yet you're not even trying to fix it!

I also cannot get this add-on to work.
I have had to follow a tedious workaround I found in the Udacity forums. I generally love Udacity, but I think it is ridiculous that a (paid) site teaching development would release an add-on that they know does not work properly (see forums).

The linked JSON page isn’t at the same origin and directory as the document."

I got "The linked JSON page isn’t at the same origin and directory as the document."

But the Chrome extension works.