Thank you for your answer !
Actually i already tried FB when i leveled my mage as Frost. Then went Fire when i began raiding, and now after Fire nerf back to frost.
Even if, on some pull, i hate waiting for a BF proc that can take way too much time to come with NT, i still find more comfortable to refresh NT versus FB because of the cast time.
But i'll be ready to change if damage makes it interesting.