It does seem to be broken in the latest version of FireFox.
Using a previous version of Firefox works.
Thanks for reporting the issue. I have verified that the issue persists only on latest version of Mozilla Firefox.
I have sent the issue for investigation and will keep you posted with the updates.
Thanks & Regards,
FWIW: I can see this also on Firefox 62.0.3 on Windows 10.
It's not a OS X issue, only.