|Toughness is how many dice you roll, Firepower or Strength is what you need to roll on EVERY die to kill that model.|
I haven't done a D10 version of this chart before, but here's the likelihood of killing a model of a given toughness with a given Str/FP. The zero results aren't really zero, of course. But I didn't want to have to show percentages out to 10 decimal places. Let's just say that zero means 'very unlikely'.
I'm thinking that the cap for Toughness should be 5 or 6, as anything beyond that gets a bit ridiculous. Whether we actually ever have 'auto-kill' weapons of Str/FP 10 is another matter.