If an eqnarray* tag is put inside the folded away part of a fold and then exported as a .html file with the html-fold package, the eqnarray* doesn't get hidden when folded in the browser (Firefox 86.0).

This appears to be due to differences between HTML and XHTML. If the file is saved as .html, the source is not properly parsed by Firefox: some tags, including the </p> after the folded equation, show up as red in Firefox's view-source:, indicating an error.

If instead the file is saved as .xhtml, Firefox throws error concerning the minimized attribute `defer` in the script tag for `texmacs_fold.js`:
<script src="https://www.texmacs.org/javascript/texmacs_fold.js" language="javascript" defer></script>
Changing this to defer="defer" fixes this.

Furthermore, the ampersands in the MathJax eqnarray* throws an error in XHTML. Changing these to `&amp;` fixes this as well.

Attached are a simple test .tm file, the export as .html and .xhtml files (these are identical in content), and a fixed .xhtml file.