Defrost uses the a/c, if it doesn't smell the same with a/c only on. The smell is coming from the blower motor. BMW's for example have a problem where the blower blade contacts the plastic housing and causes it to rub/melt.
The burning rubber smell in the tranmisson breaking apart inside.
Possibly a problem with one of your belts?
The first and easiest answer that comes to mind is that there is a problem with the cabin air circulating fan. BUT - - there is not really enuf specific information. Are you sure it is a burning RUBBER smell, or a burning PLASTIC smell, or a burning ELECTRICAL smell? All could point to different problems.
Bad heater core.
My new car did not smell like burning rubber! When I had a gate smelling like burning rubber, a rubber belt was stuck and instead of turning with a piece of machinery, was caught. I would check the belts in the front of the car and look for one with a worn or burned place on it. It could also be a wire or tube resting on an exhaust manifold or on an exhaust pipe.
A flat tire is one possibility
when changing the oil sometimes oil can be spilled on the engine block. as your engine heats up it causes this smell. it is completely normal
I advise you to perform a careful examination of your car's brake system.
Most likely the transmission fluid needs to be replaced.
Acrid
Rubber tree wood is not safe for burning in a fireplace indoors. Rubber tree would can be burned in an outside fire pit. Sometimes the fumes from rubber tree wood can be toxic.
It would depend on what is burning.