Cervical ribs are very rare and can cause thoracic outlet syndrome (symptoms in the arms). Extra lumbar ribs are generally benign and do not cause problems.
Toe numbness can be cause by local foot problems (compression of the sensory nerve in the toe) but would be characterized by numbness only on one side of the toe (unless both medial and lateral great toe nerves are compressed- very rare). The nerves that supply the foot can be compressed at the ankle or the knee and can cause numbness. Finally, nerve root compression in the spine can also cause these symptoms.
A normal MRI of the spine makes me think of the problems existing in the leg and not the back although there are instances where a small compressive lesion might have been missed by the radiologist.
If the numbness does not bother you other than occasionally noticing it and there is no pain, motor weakness, imbalance or any other neurological symptoms, it is probably OK to leave it alone. If it is disturbing, a visit to a neurologist would be the next step.
Dr. Corenman