It seems you are using Louis-Sebastien Mercier and his "Tableau de Paris" as your primary source. It also seems you misunderstood a passage.

Tel homme arrive au café sur les dix heures du matin, pour n'en sortir qu' à onze heures du soir ; il dîne avec une tasse de café au lait, et soupe avec une bavaroise : le sot riche en rit, au lieu de lui offrir sa table.

Louis Sebastien Mercier while talking about Cafés (the place, not the beverage) mentions that the population frequenting this place is often poor and idle. They came early in the day (he says around 10am) and left late at night (around 11pm) because it is cheaper to stay warm there instead of heating their own houses. He also mentions they only had coffee at lunch and a bavarois (a type of cake) at dinner. "Le sot riche en rit(...)" is just part of the following sentence where he complains that the rich man laughs at that behaviour instead of offering them a place at his own table.

So, you see he didn't mention breakfast at all.

Breakfast in Cafés were indeed a common thing for the middle class in Paris (but also in other cities). A typical breakfast in the early 19th century was probably buttered bread and coffee, fruits and maybe pastries. Others could go for cheese or a glass of wine. The poorest ate soup and bread

Croissants (wherever they came from) were not a staple of french food as they are now.

My source (in French) is from a contemporary author : E. Pouyat, Les déjeuners à Paris in Paris ou Le livre des cent-et-un, 1834 (https://books.google.fr/books?id=ewFAAAAAcAAJ&pg=PA247&lpg=PA247&dq=E.+Pouyat,+Les+D%C3%A9jeuners+de+Paris&source=bl&ots=QjwFCPUtRu&sig=STWOfkod-YPmGN-G2JPKAfiuK5k&hl=fr&sa=X&ei=Y_CSVPyBB9Kt7Aa-kYCYDg&ved=0CDAQ6AEwAg#v=onepage&q=E.%20Pouyat%2C%20Les%20D%C3%A9jeuners%20de%20Paris&f=false)

Under the FAQ, one of the questions about food deals specifically with "Continental" breakfasts.