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Backfiring in the exhaust system happens when excess, unburned gasoline accumulates in the muffler, where it explodes when the moment is right. It is caused by either an electrical problem failing to ignite the fuel mixture in the motor, or, paradoxically, by the fuel mixture failing to ignite because it is TOO LEAN. The former usually occurs under heavy load and at low speeds. When the accelerator is fully depressed and the low rpms allow plenty of opportunity and time for a maximum charge of fuel/air mixture to be drawn into the combustion chamber, the resulting high pressure on the compression stroke can raise the electrical resistance to spark formation by the spark plug to the point that there is no spark and the unburned fuel is swept out into the exhaust system. This is aggravated by anything that causes a weak spark or by the presence of alternative paths-to-ground for the high voltage sent to the spark plug, such as cracks, scratches, or dirt on the surface of the distributor, plugs, or wiring. Obviously, there are numerous potential culprits, including the most popular straw to grasp at--changing the condenser.
By contrast, the too-lean problem usually occurs at light loads and high rpms--like running down hill or during gear shifting--during which your foot is OFF the accelerator, thereby limiting the fast-revving engine to fuel it can suck through the carb's idle circuit, which is insufficient to support all of the combustion firings that the poor motor is required to make at high rpms. The quick fix is to open the gas adjustment valve (counterclockwise, possibly a full turn or two) on the end of choke rod; the real fix involves somehow permitting more fuel to flow thru the idle circuit.
So, depending on which set of conditions your backfiring occurs at, look to either the electrical system or the carburetor for the solution. Both conditions can be aggravated by leaks in the manifold connections, either by permitting air to be sucked into the exhaust system upon cool down or by blending too much air into the fuel mixture at the front end of the process.
I'm on shakier ground regarding backfiring thru the carb, but I believe that it is rarer and probably more serious. Exhaust system backfiring can give you or passersby a fright or blow the muffler apart, but carburetor backfiring can cause a fire and burn the car up! It can be caused by the valve timing being screwed up such that the combustion process, which is supposed to be contained in the head, is getting by an intake valve and being shared with the carburetor. I suppose this would be aggravated by too rich a fuel mixture, but I doubt that fiddling with the carb will fix the problem.