Is Marseille a safe place to ride a bicycle?
Marseille was voted worst French city for cyclist in 2013 (see here), but apparently they are trying to change that. Here are potentially useful links: maps of cycle lanes in the city, and a city bike rental scheme run by the city.

You should keep the following in mind also: Marseille gets very hot in the summer and is one of the worst French city for air pollution. You should also get a very good bike lock as theft is rife in some neighbourhoods.

Despite all this, I think it doesn't hurt to try it for a while using the bike rental scheme (first 30 minute is free apparently) and decide if it's for you or not.