CLOSED Why might the email plug-in succeed with Chrome and Edge but fail in Firefox?

We are using the Email plug-in for an ordinary enquiry form. This succeeds in Chrome and Edge (displays our "Thank you" message) but fails in Firefox (displays an "Oops" error message) after clicking the Send button.

Any ideas for solving this problem?

Thank you.

The problem relates to nonce verification in the form plugin so I am closing this topic and opening another.