Mozilla is part of the not-for-profit Mozilla Foundation, which launched Firefox as a rival to Microsoft's Internet Explorer in November 2004.
Since then, the browser has increased its market share to 12.9 per cent (against Internet Explorer's 83 per cent) and, through a tie-up with Google, now turns in a profit.
The appointment of Fuse comes ahead of the launch of Firefox 2 at the end of this month.
Fuse will oversee this, and other, launches. It will also run an ongoing campaign, which promotes Firefox's security and ease of use.
The agency was appointed after previously undertaking some pro bono project work for Mozilla. It reports to Mozilla Europe's HQ in Paris.