Thunderbird might be leaving MoFo to become an independent entity. Mitchell Baker discusess this in her blog and Scott Macgregor, lead engineer of Thunderbird, also chimes in for a more in-depth explanation about why Thunderbird is looking for a new home.
This could be seen as a sad moment from the community's point of view. But like all things that "are hard, but must be done," I think this is a necessary step that will benefit both Firefox and Thunderbird. I will just point you the mentioned posts above, as well as one by Ben Goodger (on renaming the Mozilla Corporation to Firefox Corporation) for more thoughts on this.