Straight lines of \vmatrix{} doesn't show up on Firefox

Created by: Mathebibel

Steps to reproduce

Open https://www.mathebibel.de/inverse-matrix-berechnen-mit-adjunkte#beispiel-1 with Firefox 89 or 90 beta (tested on macOS Big Sur, Windows 10 and Linux Mint 19)
Look at the math content below (1) Determinante berechnen: The straight lines of the \vmatrix are missing.

With Chrome, Edge and Safari everything works fine.

EDIT: I tested Firefox 88 and below and there it works. Please check the release notes of Firefox 89. Maybe you can identify the bug.

Working example

I was surprised that the following works even on Firefox 89+:

<!DOCTYPE html>
<head>
  <meta charset="utf-8">
  <meta http-equiv="x-ua-compatible" content="ie=edge">
  <meta name="viewport" content="width=device-width">
  <title>MathJax v3 with TeX input and HTML output</title>
  <script>
  MathJax = {
    tex: {inlineMath: [['$', '$'], ['\\(', '\\)']]},
    chtml: {displayAlign: 'left'}
  };
  </script>
  <script id="MathJax-script" async src="https://cdn.jsdelivr.net/npm/mathjax@3/es5/tex-chtml.js"></script>
</head>
<body>
    <h1>Testing...</h1>

    <h2>...pmatrix</h2>

    <p>
      \(
      A
      = \begin{pmatrix}
      4 & 3 \\
      5 & 7
      \end{pmatrix}
      \)
    </p>

    <h2>...vmatrix</h2>

    <p>
      \(
      A
      = \begin{vmatrix}
      4 & 3 \\
      5 & 7
      \end{vmatrix}
      \)
    </p>

</body>
</html>