The Florin was another word for the currency of the Netherlands, the Guilder. Its symbol was ƒ. It was finally replaced by the Euro in 2002 although it had long been attached to European exchange mechanisms before that.

The Dutch International Institute of Social History has a calculator for modern day Guilder value. One Guilder in 1938 had a similar purchasing power to 9.61€ today.

However during the time Anne Frank lived under German occupation the Guilder was pegged to the German Reichsmark at a fixed rate of 1 Guilder : 1.5 Rm. This wasn’t a favourable exchange rate at all and allowed German to legally plunder the Netherlands.

Edit: I put the wrong Florin/Rm exchange rate eeeeeee