what is the difference between the 1955 and 1956 thunderbird?

This is pretty decent write-up on the old t-birds. http://en.wikipedia.org/wiki/Ford_Thunderbird#1955-1957_.22Classic_Birds.22_or_.22Little_Birds.22

The original 1955 T-Bird ran very hot and became quite uncomfortable for the driver and passenger. Easiest way to spot a 1956 is the rectangular vent just ahead of both doors to dissipate that heat. Not too many other differences that I'm aware of. the 1957 was the last of the two-seaters and had tai fins. This is the most collectible T-Bird and the "F" model was even offered with a supercharger.

The 55 Thunderbird had a solid hardtop without portholes. The 292 engine was 193 horsepower with a standard transmission and 198 with the automatic. The 55 has different tail lights and the exhaust exits through holes in a bumper between the bumper and the trunk. The 55 had no wind-wings and no vents in the front fenders. The 55 had a flat horizontal bar on the steering wheel and the 56 had a deep dish wheel. The 56 had a 292 and 312 cubic in engine and also offered dual quad carburetor setup. Ford decided in 1956 to add a continental kit because the spare tire in the 55 takes up a lot of room as it lays flat in the trunk. The kit weighs and extra 75 pounds and owners said the cars would under-steer. Hope that answers some questions.

The 1955 had a 6 volt electric system, the 1956 had a 12 volt system.