Checkbox events are not correctly triggerd on Firefox [withWorkaround]

### bug report

### What is the current behavior?

Checkbox events are not correctly triggerd. ex : **active** props from onChange from FormSpy is always undefined.
(see sandbox)

### What is the expected behavior?

- active value should be the name of the input.

### Sandbox Link

two checkbox inside the sandbox link : one is called bug, one is called workaround: test both
https://codesandbox.io/s/react-final-form-field-level-validation-example-forked-y7ome?file=/index.js

### What's your environment?

### Other information