Looks like yet another issue with Codepen and Firefox. This time I’m more convinced that it is the missing allow-same-origin on the sandbox attribute for the iframe that is (likely) the issue.
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It works on other live editor sites that have that attribute value.
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It works on Codepen in Debug mode, which is not using an iframe (Change View > Debug mode).
Edit: I filed another bug report to Codepen. I’m pretty sure this issue is the same as we have with localStorage and the test script (thread link).