Ubuntu Linux developers (particularly Canonical) want to switch to Chromium with their latest reason being the Ubuntu Touch stack is being powered largely by Chromium/WebKit so they now have a vested interest in its success. As mentioned yesterday, for those preferring Firefox, the Mozilla browser will continue to be offered through the Ubuntu package archives but this discussion is simply a matter of what's the default web-browser for the Ubuntu desktop.
Expressed shortcomings of switching to Google's Chromium open-source web-browser is that data migration from Firefox isn't too obvious, extensions don't migrate between browsers, Chromium isn't supported on all architectures (e.g. PowerPC), the browser doesn't work with the Orca screen reader and doesn't integrate well for accessibility reasons, there is no native PDF plug-in, and Chromium is said to have worse performance under memory pressure. There were also some concerns expressed about differences with WebApps in Chromium.
When asked by an IRC listener about porting Chromium or Firefox to Mir, it was a question without an answer by the Canonical engineers present in the session.
It looks like the switch to Chromium will happen in the name of a better user experience for the desktop with Chrome/Chromium now arguably surpassing Firefox in its features and performance while pushing Chromium as the default leads to a more consistent experience across Ubuntu form factors from phones/tablets to the desktop. Officially, as the next step, a discussion will be started soon on an Ubuntu mailing list to solicit a more broad range of feedback on switching from Firefox to Chromium.
For those dissatisfied by this likely change, it will just be a matter of sudo apt-get install firefox on new installations to go back to the Gecko experience.