Mining warning not showing in Firefox?

See my post in this thread.

I'm posting it hear also since my main concern is why is Firefox not showing the warning? In Chrome I get the below message warning, but nothing in Firefox. Same site visited at the same time, and I tried it multiple times. Only Chrome shows it. Also as the message says pressing "Cancel" prevents it? Seems like it should be backwards. Press "OK" to stop it. Maybe it should read "Allow" or "Block/Deny".

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