Probably Catherine de Medici - wife of Henri II of France - who had a reputation for poisoning her enemies.
Another contender might be Lucretia Borgia, but the Borgias neither came from Florence nor did Lucretia marry a Florentine husband.
Thank you kettledrum That was one of the names suggested and I think it fits- Catherine de Medici I mean. Lucretia was also put forward but as you say no connection with Florence.. Thanks again for your help.