As far as I can tell, it is the first page. I did a lot of testing with the calibre driver and utilities plugin to see what happens. That was a while ago, but a quick test of this with FW3.2.0 suggests it hasn't changed.
I also dropped the epub I changed into ADE. It used the first page for the cover as well. That suggests it is part of the RMSDK that is doing this.
If you sideload as a kepub, the rules are different. In that case it uses the epub3 standard to work out the cover.
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