chrome/webkit can run JS under the file:// scheme also, but it cant load the images because it treats it like a cross origin request. In order to load them, you need to run a local server (I find this preferable even when using FF since it more closely replicates the environment for a real deployment in the future if the sketch is going to be web based)
It depends on what browser your running!
ps.
I wasn’t trying to say that you were wrong, just that the answer you provided lacked context (especially for a beginner) to understand why this would work in FF vs another browser! In the future, even a a few words to say why you linked to a post would be super helpful for a beginner.