Firefox: Uncaught (in promise) TypeError: Failed to fetch (How to delay form submit until javascript is executed?)

Question

I have this solution integrated (a simplified example is below)

<form action="/action_page.php" onsubmit="downloadPdf()" >
  <label for="fname">First name:</label><br>
  <input type="text" id="fname" name="fname" value="John"><br>
  <label for="lname">Last name:</label><br>
  <input type="text" id="lname" name="lname" value="Doe"><br>
  <label><input name="privacy" type="checkbox" required> I agree to the privacy policy</label><br>
  <input type="submit" value="Submit">
</form> 

<script src='build/pdfmake.min.js'></script>
<script type="text/javascript">

        function downloadPdf() {

          // create PDF
            pdfMake.fonts = {
                // my fonts here
            },
           let docDefinition = {
                // my definitions here
            };
       pdfMake.createPdf(docDefinition).open();
        }
</script>

So, form submitting does two things, executes PDFMake and submits the form. It works in Chrome and Edge without issues, but in Firefox I am getting this error

Uncaught (in promise) TypeError: Failed to fetch (url: "http://localhost/site/assets/fonts/myfont.woff")

Do you maybe have idea how to solve this?

What's funny, when I remove the privacy checkbox from the form it also works in Firefox. But the privacy checkbox is mandatory these days, so I can't remove it.

I think form submiting should be delayed until javascript function downloadPdf() is executed and then submit the form. Which works automaticaly in Chrome and Edge, but not in Firefox. But I am not sure how to do it.