Given the fact that Brussels is not the biggest airport, I guess 1h30 is more then enough. I'v done enough walks in the past through the terminals while waiting on departure and it took me less then 10 minutes to make it from one end of the terminal to the other.

As you're arriving from a non-Schengen destination, you will arrive in terminal B. Florence on the other hand is a Schengen destination and will depart from terminal A. Unfortunately I have no idea of the connection procedure / time needed as Brussels is my final stop, being from Belgium. But when leaving the aircraft on arrival in terminal B, I know there are signs for flight connections to terminal A / T before the passport check but I have never taken them. Biggest bottleneck at Brussels airport is usually the passport control, especially if the automatic passport gates for EU-passports are (once again) out of order.