The Mozilla Foundation stopped active development of the Thunderbird stand-alone email client in 2012, a year before Edward Snowden's revelations about mass email interception by spy agencies sparked an exodus from webmail platforms.
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are many good reasons to use standalone email clients, but for Americans one of the most compelling is the absurdly outdated Electronic Communications Privacy Act of 1986, which treats any file left on a server for more than six months as "abandoned" and accessible to law enforcement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!). That includes all your Gmail previous to June 2015. Really. All of the efforts to reform ECPA have died on the vine, because law enforcement loves this creaking piece of legislation.
I donated to the Mailpile crowdfunding effort to see if we could bootstrap a new Thunderbird alternative with cryptographic privacy by design. They're had some promising betas, and I'm looking forward to getting a working version someday.
Mozilla and the wider Thunderbird community have not provided any updates on how many Thunderbird users there are today, or how many downloads of the client, or what kind of usage the application sees.
And for Mozilla itself, focus has largely switched to developing its core Firefox browser for more platforms, and generally making Firefox more of a business, integrating with Yahoo and Google to generate revenues around search ads.
When Mozilla passed Thunderbird development on to a volunteer-led community in 2012, it committed itself only to providing "extended support releases" focused only on security and maintenance updates mainly aimed at large organizations who use Thunderbird. In that regard, for Thunderbird and its users, this is potentially a rough turn, but it's not really a surprising one.
Mozilla Wants To Split Off Its Thunderbird Email/Chat Client, Says Mitchell Baker Memo