Read the full interview. Netscape was owned by AOL back then and the motivation for Gecko was to have an HTML engine that could be embedded in AOL's software. Firefox was just one browser based on that engine, but became the primary focus once its popularity rose.
Camino was another browser based on Gecko, but died in 2011 when Mozilla killed off support for embedding: http://caminobrowser.org/blog/#mozembedding