First off, there is no such thing as being "toed in at the top". Toe-in means that the front edge of the wheel is more inboard than the rear edge. Camber is probably the term you're looking for - negative camber means the top of the tire is more inboard than the bottom.
As far as what the cause is, it really depends. First off, is it the front or rear? If it's the front, I suppose a bad strut mount could cause the top of the knuckle to be tilted inboard, thus causing too much negative camber. Our cars use a MacPherson strut design, meaning that the strut is an actual structural member of the suspension. In theory, a bent strut could cause too much camber. Likely? I seriously doubt it.
If it's in the rear, then there is now way that a bad strut would cause camber issues. At all.
I'm gonna go with Accam's Razor on this one, and say that the most obvious solution is probably the correct one - the dude at Firestone probably doesn't know what the hell he's talking about.
Anyway, if you could give us some more info and/or pics, we could probably help you out more.