The publication of the Netscape source code under free software license and the launch of the Mozilla project constitute a pioneering initiative in the field of free and open source software. However, five years after the publication came years of decline. The market shares rose again after 2004 with the lighter Firefox browser. We propose a case study covering the period from 1998 to 2012. We identify the factors that explain the evolution of the Mozilla project. Our study deepens different success factors identified in the literature. It is based on authors’ experience as well as the abundant literature dedicated to the Netscape company and the Mozilla project. It particularly highlights the importance of the source code complexity, its modularity, the responsibility assignment and the existence of an organisational sponsorship.