Cory Doctorow reports on Mozilla dropping/freeing/spitting off the Thunderbird e-mail client and explains why you should use a stand-alone mail client rather than webmail. (I use Sylpheed, myself, for its support of MH-style folders, I like my mail in a directory tree I can run find and grep over.)
Thunderbird -- which I use for my own email -- is creaky and poorly maintained, something that is tacitly admitted by Mozilla Foundation CEO Mitchell Baker in her memo, where she describes how trying to balance the needs of Thunderbird and those of Firefox often puts the two teams at cross-purposes. Baker doesn't describe exactly how Thunderbird will stand on its own, but I've heard reliable internal rumors that a new nonprofit entity is likely to be stood up to maintain and advance the project.
I live in my email (I hate instant messaging, and relying on platforms like Google or Facebook to maintain your messages is a terrible idea, since both are liable to squeeze their users if it is commercially expedient to do so). As a hardcore Thunderbird user, I'm glad to see something happening with the project. I would contribute to a nonprofit for maintenance and advancement of Thunderbird, and I'm sure I'm not the only one.
There are many good reasons to use standalone email clients, but for Americans one of the most compelling is the absurdly outdated Electronic Communications Privacy Act of 1986, which treats any file left on a server for more than six months as "abandoned" and accessible to law enforcement without a warrant (no, really!).