I've picked up cars at the Florence airport without incident several times. This time on a September 2014 trip I got a ticket - received seven months later as expected - for a violation 10 minutes after picking up the car. I expected speeding or a ZTL mishap in a village, but no: apparently I entered a bus lane. I've never seen this discussed on the forum, so watch out for this. Insult to injury -- you can get 30% discount on your fine if you pay within five days of notification (first you get notice of charge for rental agency giving them your contact info, then notice of ticket with no info, then three months later the actual ticket you pay. Not much chance of the discount! Total for everything: 155 euros.
Sorry to hear about your bad experience driving in Florence.
This is just one example or reason why some of us here recommend not messing around with rental cars in Florence. Lots of ways you can get blind-sided by legal violations that cost you money, sometimes significant amounts.
There is a big difference between ZTL and bus lanes: while ZTL is not enforced by night (*) and on festive days, bus lanes are always reserved to buses and you will get a fine at any time if you use them.
A few bus lanes are quite easy to enter if you are not careful: via de' Panzani going direction Duomo, via Valfonda (reserved to buses in both directions). Bus lanes are always marked by no-access signs at their beginning and yellow stripes on the ground.
(*) Well, this is not completely true; in summer months ZTL is enforced also on late night.
Charles Laughton as Inspector Javert:
Right or wrong, the law is the law and it must be obeyed to the letter.
["Les Miserables", 1935 Twentieth Century movie adaptation of the Victor Hugo book, http://www.imdb.com/title/tt0026725/].
ZTLs and bus lanes have "no access" signs (empty red circle on white background), and a supplementary sign stating exceptions (residents, public transportation, free times, etc.). I understand that interpreting the exceptions may be difficult, but the no access sign should be already a warning. Also, you get an automatic fine only if you cross a detection camera - devices mounted on big poles that should be evident enough.
How many times can it be said here...rental vehicles are mostly pains in the arse!
Camera enforcement is increasingly the norm across Europe. The advice is keep an eye on the signs and know what they mean. If it is any consolation a friend got caught out on a local to them camera at 0200. Instant £100/140€ fine.
Cameras do not blink, which is why local authorities like them. They catch out everybody.
Bus lanes in Italy are marked in yellow markers on the side, and they have a sign indicating that (a bus on a blue circle).
Taxis and government cars are often allowed to use them, which is why some people miss the cues and enter them as well.