Unreliable drag-n-drop in Firefox?

Question

We are experiencing significant problems with javascript drag-n-drop in Firefox 56. The symptom is that drag-n-drop stops working across all tabs/pages - not just our tab.

Commenting out the call to setData() in this jsfiddle makes the problem occur reliably. Our code is calling setData; however, after a few drags it stops working.

https://jsfiddle.net/5jobd9tw/27/

function createLi (number) {
let li = document.createElement('li');
li.draggable = true;
let span = document.createElement('span')
span.appendChild(
    document.createTextNode('element ' + number)
)
li.appendChild(span);
li.addEventListener('dragstart', function (event) {
    log('element', number, 'dragstart event');
    event.dataTransfer.setData('text/plain', 'foobar');
    // event.dataTransfer.setData('text/json',
    //  JSON.stringify({ foo: 'bar' })
    // )
    event.dataTransfer.setDragImage(noisyCanvas(), 50, 50);
});
li.addEventListener('dragenter', function (event) {
    log('element', number, 'dragenter event');
});
li.addEventListener('dragleave', function (event) {
    log('element', number, 'dragleave event');
});
li.addEventListener('dragend', function (event) {
    log('element', number, 'dragend event');
});
return li;
}