We just upgraded to Lotus/IBM Notes 9 "Social Edition" and there is this Discover tab (which is basically the product just advertising itself) which always opens on start up. Normally Lotus Notes will restore my tabs on startup, and it still does that, but it will always open this Discover page too. I'd rather it doesn't!
The right-click / set as home page trick did not work for me. Notes would still open the "Discover" page regardless of what other page I told it I wanted as default.
I managed to fix the problem by manually editing
notes.ini file, setting
$DISCOVER_HOMEPAGE=1. At the next restart of the Notes client, the "Discover" page was finally gone.