See [http://en.wikivoyage.org/wiki/Paris#Get_in Wikivoyages on getting to Paris].
Note that Gare du Nord is about a 20-25 minute walk from the Mozilla office, so depending on the weather and what you're carrying, you might choose to walk from Gare du Nord to the office (alternatives: Metro with transfer, taxi). But if you do, you should definitely consult a map first and probably have the map with you.