The Northerners did not like the Missouri compromise because they had less power. The South had more states than the North. To fix this, what used to be a part of Massachusetts, Maine, it counted as a free state. That made the power stable.
The Missouri Compromise was when Missouri wanted to become a slave state. That became an unbalanced amount of states that had slaves. So to keep it balanced, Maine was added as a free state. The second part of the compromise was that there was a line drawn above the Arkansas territory that said anything above 36/30 would be a free state and anything below 36/30 would be a slave state.
The Southerners favored the Missouri compromise because they got another slave state. This gave them more power because there were now 12 slave states and 11 free states.