[Square] The payment form fields are not loaded properly in Mozilla Firefox: «Cannot read property 'postMessage' of null»

Extensions Square
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1

When checkout is rendering.
An error display in javascript console related to credit card fields
Check this image:

Also that error happens in Mozilla Firefox and fields don’t load properly

2

Unable to reproduce the «that error happens in Mozilla Firefox and fields don’t load properly» issue.
I use:

  • The latest version of the Square extension: 1.3.7.
  • Firefox 56.0 on Windows 7.
  • A localhost instance of your website.

POST https://connect.squareup.com/v2/v works correctly:

Interestingly, there is a similar issue «Invalid postMessage origin: https://connect.squareup.com» in Google Chrome.
It should not break anything, but I have reported it to the Square support: https://www.sellercommunity.com/t5/Troubleshooting/Connect-API-v2-Invalid-postMessage-origin-https-connect-squareup/m-p/51854