Florence never disappoints me, even on a rainy day. Actually the rain in Florence never lasts too long (at least from my experiences).
Once I was in Florence and it suddenly started to rain. I was grounded in the hotel room, worrying if it would last for rest of the day. It was in the afternoon, and I would still like to go out for shopping, dinner, and of course walking around. Just in less than one hour not only the rain stopped, but also the sun shone behind the clouds putting the whole city in a reddish glow.
So in case you are caught by a rain in Florence, I say don’t worry about it. If it really lasts long, then sip a coffee and relax. Tomorrow is another day!~