There's been discussions since May about switching from Firefox to Chromium, after similar discussions have taken place during earlier Ubuntu Linux release cycles. However, Jason Warner of Canonical announced today that the decision has been made to stick with Firefox for Ubuntu 13.10.
The primary concern raised was with regard to Ubuntu's "Web Apps" and the changing APIs around it. There's work towards a more self-contained Web Apps layer, at which point they may more easily switch to Chromium as the default browser.
The new hope is that for Ubuntu 14.04 LTS they will switch from Firefox to Chromium by default. Until then, Google's open-source web-browser will still be available for installation from the Ubuntu repository.
The Ubuntu desktop browser discussion announcement from earlier today can be found on the ubuntu-desktop list.