I'm a theatre historian, so my knowledge of censorship in the 18th century has to do with performance more than it does with literature. However, I can at least give you some idea how censorship worked in France until someone more well-versed about Voltaire and the history of the novel can step in.

The French form of censorship was quite haphazard. In England, for example, parliament passed the Licensing Act of 1737, which essentially required that all dramatists submit their plays to the Lord Chamberlain for approval before they could be performed. This was a specific law that lasted over two centuries (with some modification). In France at this time, there was no such act that we can point to, no laws that can be called "laws" as such. Instead, we see patterns of royal injunction, moments when the King stepped in and excised a work for one reason or another.

When Molière had trouble putting on his play TARTUFFE in the 17th century, it was not because he was disobeying any laws. It was because Molière had garnered some powerful religious enemies who found the play to be libelous against the clergy. They were able to persuade Louis XIV (well, really Louis's mother who then persuaded Louis) to forbid the play subsequent performances, and Louis acquiesced because, at the time, he didn't want to have to bother with the angry clergy, especially over something so insignificant as a play.

In the eighteenth century, the theatres themselves were more often the ones whispering in the King's ear to censor their rivals. At the beginning of the century, the Comédie Française had a monopoly on all spoken drama (comedies and tragedies written in French) and were quick to shut down any performance of play spoken in French outside its walls. But, we find cases where their monopoly had little reach -- particularly with the Italian troupe who performed plays in French at the Hotel de Bourgogne. They got away with it because the King liked them and was willing to overlook the Comédie Française's monopoly on their behalf.

So, while not directly answering your question, these two examples give you a sense of how censorship worked in France. It depended on political leverage, not law. If Voltaire found himself in hot water, and even denied the authorship of Candide, it was because he knew it would put him in hot water with powerful people, people who had connections with the King. The book is quite unforgiving with religious types, and Voltaire already had quite the reputation for being a blasphemer. When he went to Geneva, it was likely to escape the reach of those who could destroy him. He's not the only philosophe who lived and wrote in Geneva. There you will also find Jean-Jacques Rousseau.