Not surprisingly, the major topic on which Frederick Douglass spoke to audiences was slavery. In the time after he escaped from slavery, Douglass's public life was mainly dedicated to the cause of abolition.
In his speeches, Douglass talked about slavery in a variety of contexts. However, slavery was always at the heart of what he was discussing. So, when he talked about his attitudes towards the Constitution, he talked about how he felt about the fact that it allowed slavery. When he talked about the Civil War, he did so because of the impact it would have on slavery.
So the short answer here is that he was an abolitionist and talked about slavery in most of his speeches.