(I'm making this a separate post because I don't want any distractions in the other one.)
If they had done it correctly, it would have been invisible and it would not have advertised anything to firefox users. That is the purpose. It showing up the way it did was an oversight.
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How many of those Firefox patches are advertising references to then-popular media as opposed to new features, security fixes, and so forth?
Again. Purpose.
That's not answering why it would be wrong even if it was about:blank.
(I'm making this a separate post because I don't want any distractions in the other one.)
If they had done it correctly, it would have been invisible and it would not have advertised anything to firefox users. That is the purpose. It showing up the way it did was an oversight.