Sources say that it doesn't mean anything, given the fact that it's Dr. Seuss (Theodor Seuss Geisel), who often uses made up words in his rhymes. Seuss’s stories are filled with rhyming tongue twisters and nonsense words, quotes a PBS.org article.
The phrase Fahoo fores dahoo dores sort of give the song a 'Latin-ish' sound, similar to other Christmas songs like Adeste Fideles.
Welcome Christmas is the song that was sung by all the "Whos" in "Whoville" in Dr. Seuss' How the Grinch Stole Christmas. The lyrics are meant to imitate classical Latin, according to a Songfacts.com contributor.
Besides the books, Dr. Seuss works have provided the source for eleven children's television specials, a Broadway musical and a feature-length motion picture. His honors included two Academy awards, two Emmy awards, a Peabody award and the Pulitzer Prize.
Know more about the life of Dr. Seuss here.
For interactive games, contests, and information on Seuss and his works, visit www.seussville.com.