Answer:
They wanted to secede over the issue of slavery.
Step-by-step explanation:
Unlike the Unionists who believe that the United States of America should remain together as one larger entity, the fire-eaters, however, believe in the secession of the United States. Fire eaters also favor slavery
Hence, in this case, the correct answer is, the fire-eaters differ from the Unionists in the sense that "They wanted to secede over the issue of slavery."