Steps to Reproduce

form_submit.zul

<zk xmlns:w="client" xmlns:n="native">
    <n:form action="target.zul" id="testForm" method="POST">
        <vlayout>
            <button label='JAVA: Clients.submitForm("testForm")' onClick='Clients.submitForm("testForm")'/>
            <button label="JS: zAu.cmd0.submit(testForm)" w:onClick="zAu.cmd0.submit(testForm)"/>
            <button label="JS: testForm.submit() - workaround" w:onClick="testForm.submit()"/>
        </vlayout>
    </n:form>
</zk>

target.zul

<zscript><![CDATA[
    System.out.println("received on " + desktop.getId());
]]></zscript>

click the first or second button

Current Result

FF will post the form twice

The server side will receive both requests (logged on target.zul):

received on z_kph
received on z_lph

potentially related:
https://stackoverflow.com/questions/49587933/firefox-doesnt-preventing-dispatched-submit-event#49592762
https://github.com/facebook/react/issues/12639

Expected Result

the form should be submitted only once

Debug Information

both a 'submit' event is dispatched and the submit() function is called
FF posts the form for both (potential browser bug/difference)
https://github.com/zkoss/zk/blob/v9.1.0/zk/src/archive/web/js/zk/dom.js#L76-L77

maybe related:
deprecated usage of Event.initEvent

https://github.com/zkoss/zk/blob/v9.1.0/zk/src/archive/web/js/zk/dom.js#L2662

Workaround

As implemented in the 3rd button simply invoke the native form.submit() function directly

From java: Clients.evalJavaScript("testForm.submit()");