Douglass's wirtings were almost entirely about the abolitionist cause, and how wrong slavery was. Therefore, all we are able to see of his philosophy is his ideas about slavery, and any matter that can relate to it. Even with this restriction, it is possible to see similarities between Douglass's philosophy and Emmerson's and Thoreau's.
All three had the belief that men should be free at the core of their thoughts. Civil Disobedience by Henry David Thoreau is all about how the individual should be free to govern themselves based on their own ideas of right and wrong, and not follow someone else's consience. Douglass is mainle concerned with securing more basic libertys for his people, the African American slaves. They have no right to vote or participate what so ever in the government that rules over them, not to mention that they are considered property instead of people.
Where Emmerson and Thoreau had no real interest in organized religion or any conventional ideas about God, Douglass accepted Christianity. However, he was not afraid to speak out against the church when he thought it was wrong, unlike Puritans and other such people with absolute faith that their church is infallible. He openly denounced Christians with proslavery opinions and beliefs as hypocrites, which was basically calling out every Christian in the South (bold strategy, my friend) (Trolard).
Another difference between Douglass and the Trancendentalists is the Douglass never gave much thought to nature, while the Trancendentialist were focused on nature as the way to fix the evils of society. Douglass prefered to work within society to fight for his cause, instead of seperating himself from people (which would have made no sense at all, because if you are wanting recognition from a government you really should not ignore that government). He wanted to change society so it would recognise his most basic rights, while the Trancendentalists wanted to change society so it would accept that they just wanted to be left alone.
Another difference is that the Trancendentalists had a certain naievity to them, which Douglass had no share of. Thoreau believed that if people were left to govern themselves, they would do a good and fair job of it. Douglass had seen what evils people were capable of first had in his time as a slave, and thought that government was necessary to control those evil impulses from being carried out. He still had his scruples about the government, calling our Independance Day "a sham" (Douglass 337), but he sees that it is the best means to his end.
Douglass, Frederick. "The Meaning of the Fourth of July for the Negro." Comp. Jeffrey D. Wilhelm, Ph.D. and Douglas Fisher, Ph.D. Glencoe Literature. American Literature ed. Columbus: McGraw-Hill Companies, 2009. 97-99. Print.
Thoreau, Henry. "Thoreau's Civil Disobedience - 1." The Thoreau Reader. Web. 25
Jan. 2012. http://thoreau.eserver.org/civil1.html.
Trolard, Perry. "Narrative of the Life of Frederick Douglass, An American Slave, Written by Himself." In Barney, Brett, and Lisa Paddock, eds. Encyclopedia of American Literature: The Age of Romanticism and Realism, 1816–1895, vol. 2, Revised Edition. New York: Facts On File, Inc., 2008. Bloom's Literary Reference Online. Facts On File, Inc. Web. 8 Feb. 2012. http://www.fofweb.com/activelink2.asp?ItemID=WE54&SID=5&iPin= EAmL0671&SingleRecord=True.