If an opponent loses life but Florian leaves the battlefield before your postcombat main phase begins, its last ability doesn't trigger.

You get a postcombat main phase even if no creatures attacked during a turn. Florian's last ability will trigger.

You must pay all costs and follow all timing rules for cards you play with Florian's last ability.

Damage dealt to a player causes that player to lose that much life.

If an opponent loses life and subsequently loses the game before your postcombat main phase, Florian's last ability counts that loss of life.

If you somehow have more than two main phases in a turn, each main phase after your first one is a postcombat main phase, and Florian's last ability triggers at the beginning of each of them.

Florian's ability checks only how much life opponents lost during the turn, not by how much their life totals decreased compared to the start of the turn. For example, if an opponent lost 2 life and then gained 8 life before Florian's ability resolves, you'll look at the top two cards of your library.