Ask the expert!
There are tons of people who know French tennis better than I do, you know, I've been following it for a couple of years only. The others just don't post as much as I do.
Marc Gicquel smokes or still used to smoke not so long ago, I haven't seen it myself, but I've heard it more than once from people I trust.
Grosjean used to be an occasional smoker, but I doubt he still is. That's sure too, Wertheim (?) mentioned it once in an article, he had seen Seb smoking in the hotel lounge and was very shocked. And Seb also says it in the book he published together with Arnaud, "Passing Potes", they say that they smoked after their semi-final in Melbourne.
Escudé smoked and he still smokes a lot, but he doesn't play anymore. I don't know for the young generation, it's more the older generation, I think.
I think CoocooCachoo says he saw Faurel smoke, but he's a Challenger player. Olivier Mutis smoked, that's sure, but he hardly plays anymore.
A lot of French coaches smoke, I always find that quite shocking when I see them together with the players at tournaments. Thierry Champion, the former coach of Paulo and Monfils, for example.
There was a thread about it in GM, I'll look for it.