We are wondering if a .net containing the exact keywords that we want to promote can get to the top of Google.fr against a .fr or .com other powerful websites. has anyone got any examples of a .net at the top
No proper reason can be given. If you do your proper On-page and Off-page optimization then there is no need to worry about .net or .com or whatever it is.
You should concern yourself less with domain extensions and more with quality content + on-page and off-page SEO.
It certainly depends on many factors. But you generally have a better chance with a .com for highly competitive keywords.