Powered by Mozilla, #2
Second, yes, right, a "Powered by Firefox" logo is not desirable at all. It was in my original post only because I was thinking at loud. But "Powered by Mozilla" remains uninformative to end-users. If it's meaningless to end-users, it's value to embedders is close to zero. So my strong preference goes to a "Powered by Mozilla, the technology inside Firefox" or "Powered by Mozilla, just like Firefox" or anything that emphasizes the two names; it is then in my opinion a much better choice.
Last point, I never said Mozilla Corp’s not doing the right thing. I even applauded the "Powered by ..." initiative loudly. I only said such an initiative will fail if the proposed logo gives no return value to people using it. Apparently, other embedders agree with me. Like it or not.