To be specific;

Way Out

1. Check bag in FLL through to YYZ.
2. Domestic leg FLL-ATL, you MAY be asked for your passport at FLL, but you can do online check in, so it's not guaranteed (make sure you have it).
3. Change in ATL, you WILL be required to show your passport before boarding for YYZ.
4. Arrive in YYZ, hand over completed boarding card and passport to Canadian Customs Agents, welcome to Canada.


Way Back

5. Depart YYZ - TAKE NOTE - You will check in and then walk to the US departures area, it is a long walk, give yourself time. You must complete a Blue customs declaration at this point.
6. You then enter a US Customs and Immigration area where your Passport and Customs forms are checked.
7. Once you cross the line past the Customs and Border Protection Agent you are effectively on US soil (but still in Toronto Airport).
8. Once you board and take off from YYZ it's the same as being on a domestic leg to MEM.

For some reason the Canadian based CBP agents seem to be even more charmless and uncivil than the rest of them, so don't give them an excuse to fark you over. Get your customs declaration complete, have all your stuff together and smile until you are through.