I scan the QR code, and WhatsApp Web works fine.
I close the tab, some time passes (I don't even restart Firefox), I open
web.whatsapp.com again, and it shows the QR code all over again. I have to scan it with the phone every time. On Chrome I do it once on the device and that's that.
Anything I can do to fix this? I'm on macOS, Firefox 61.0b12 (64-bit).