This looks to be an issue specifically with Codepen, Firefox, and localStorage. The test uses localStorage and we have implemented a test for it to make sure the user is not blocking it. This is why you see the alert message.
Uncaught DOMException: The operation is insecure when trying to set something to localStorage on Codepen in Firefox.
Here is a simple test that fails on Codepen in Firefox
It fails with modernizr as well.
Works fine on jsfiddle, repl.it, and codesandbox
Not really sure what is going on yet but one thing I did notice was the
sandbox attribute for the iframe on Codepen does not have the
allow-same-origin value which the other sites do have. Not sure if it’s the issue though.
sandbox="allow-downloads allow-forms allow-modals allow-pointer-lock allow-popups allow-presentation allow-scripts allow-top-navigation-by-user-activation"
sandbox="allow-modals allow-forms allow-scripts allow-same-origin allow-popups allow-top-navigation-by-user-activation allow-downloads"
sandbox="allow-forms allow-pointer-lock allow-popups allow-same-origin allow-scripts allow-modals"
sandbox="allow-forms allow-modals allow-popups allow-presentation allow-same-origin allow-scripts allow-downloads"
I don’t think there is much we can do about it on our end. It would seem to be an issue Codepen has to fix.
Edit: I send Codepen support a message about this.
Edit2: I got a response back from Codepen and they are looking into it.