URL: https://codesandbox.io/s/mornj9oymx

Browser/Version: Firefox 60.0

Operating System: Windows 10

Problem type: design is broken

Steps to Reproduce

1. Navigate to: https://codesandbox.io/s/mornj9oymx
2. Hover an menu item: The star icon will appear.
3. Hover the star icon:

Expected Behavior:
When hovering the star icon, it will become bigger and more visible.
When clicking the star icon an event is fired which writes a console message.

Actual Behavior:
In Google Chrome and in Microsoft Edge it works as expected (as long as the menu item is not in selected state).
In Mozilla Firefox hovering the star has no effect. Clicking it doesn't trigger an event (except the one managed my Office UI Fabric itself).

Screenshot