Ali Baba, Robin Crusoe and Friday are allusions to characters from other books, books that were favorites of Dickens, but in the novella they are childhood favorites of Scrooge.

Ali Baba comes from The Arabian Nights. Next to Shakespeare and the New Testament, The Arabian Nights is the book most frequently alluded to in Dicken's writing.

Robinson Crusoe  and Friday are characters from the novel, The Life and Times of Robinson Crusoe, a novel by Daniel Defoe

When the ghost of Christmas Past guides Scrooge to his school, they see

" A solitary child, neglected by his friends, is left there still." (pg 29 - Stave Two)

This solitiary child is Scrooge.  The ghost touches his arms and points to Scrooge's younger self who is reading.  That is when Scrooge sees

"...a man is foreign garments, wonderfully real and distinct to look at:.." (pg 30 - Stave Two)

This was Ali Baba.  Ali Baba is a fictional character from Ali Baba and the Forty Thieves, a medieval Arabic tale.  It is the book that the child Scrooge is reading.  Scrooge says,

"One Christmas time, when younger solitary child was left here all alone, he did come, for the first time, just like that." (pg 30 -Stave two)

Robin Crusoe is really Robinson Crusoe.  He and Friday are fictional characters from the book Robinson Crusoe by Daniel Defoe.  Robinson was stranded on a desert island for 28 years and  Friday was the native that Robinson saves on the island and who becomes Robinson's servant.  These are all fictional characters in books that Scrooge read when he was left alone at Christmas. The memory of his lonely childhood Christmas made Scrooge regret throwing the ruler at the caroler the night before.   

When Scrooge was a boy, these were his only friends during Christmas.  The other boys had gone home, and all he had were the characters from his books.  He would get lost in his books, and these characters became real to him.