Script not loading even from the correct path

Question

I have the following HTML on my webpage:

<script src="https://cdnjs.cloudflare.com/ajax/libs/paper.js/0.12.15/paper-full.min.js"></script>
<script type="text/paperscript" canvas="paper-back" src="paper-back.js?v=<?php echo filemtime('paper-back.js'); ?>"></script>

I am using Firefox and it keeps saying at the bottom left the following message:

Read for cdnjs.cloudflare.com

I thought that it was a problem with cloudflare but all other pages load without any issues. This one also starts loading just fine if I remove the line:

<script type="text/paperscript" canvas="paper-back" src="paper-back.js?v=<?php echo filemtime('paper-back.js'); ?>"></script>

The file paper-back.js loads very quickly when I visit it in the browser. So, it is not a loading issue. Something else is causing the problem.

What could be the possible reason for this issue?

Thanks.