It all depends.........
Milano Malpensa Airport to Firenze is about 3-4 hours, depending if you drive
like an Italian or an American. If you're leaving from downtown Milano -
good luck - as many times as I've done that I still get lost.
Are you planning on sightseeing in Genova or Pisa or was that just your
route? I prefer Milano to Firenze via Bologna on A1 - I may be wrong, but it
always seems faster on that route. Although you'll probably encounter a
little more traffic from Milano to Bologna - the autostrade is 6 lanes on
that section and traffic flies. I agree with Leonardo Boselli on Genova.
And where in Firenze will you be going to? Firenze Nord exit is the first
one you will come to, but I think it's worth the few extra minutes to go
around to the Firenze Certosa exit - as the drive into downtown Firenze is so
easy from that exit.
Just stay out of the inside lane on the autostrade if you're in a 9 passenger