For weeks now developers have been talking of making the transition in Ubuntu 13.10 of going from Firefox to Chromium. Among the reasons this is being considered is that Chromium is being used on the Ubuntu mobile front, Chromium has become just as fast (or faster) than Firefox, and the features are also very competitive. Firefox will continued to be offered through the Ubuntu package archives, but it wouldn't be installed by default.
There's now a default browser follow-up discussion that's been started on the project's mailing list. While no official decision has been communicated yet, it still looks like the switch from Firefox to Chromium will happen for Ubuntu 13.10 in October.