My take on this...
I'm with other guys on 1.5 mm holes, but I thought of something more advanced (should I say challenging?). How about tiny holes < 1mm
around the perimeter of a button. Since metal buttons will eventually be flat it might be easier to make the drillings using carbide drill bits - the same type used for PCBs. This way there are no rods to worry about even though the buttons will not exactly be water proof.
I guess this is my way of saying that I'd like to be one of the "inquisitors" for a test batch that Firefox might get.