Mozilla has been rapidly pushing out new versions of the open-source email client Thunderbird lately, including version 13 just one month ago. The cycle's come to an end, however: Mozilla has announced it will be stopping development of the application because "continued innovation in Thunderbird is not a priority for Mozilla's product efforts." The company won't be leaving the application's more than 20 million users stranded, however. On November 20th Mozilla plans to release one more version, bringing complete feature parity to both it and the Thunderbird ESR (Extended Support Release) version. Both branches will receive security updates every six weeks thereafter for the duration of the ESR lifecycle, which is currently capped at one year. The developer community at large will be able to participate in further development of Thunderbird moving forward, however, with Mozilla pledging to provide resources for said efforts.

In a recent blog post, the Mozilla Foundation's Mitchell Baker admits that many users have moved to web-based email clients in recent years, lessening the need for a true native client like Thunderbird. She also writes that "driving innovation through other offerings seems a natural choice" — perhaps we'll see the results of that innovation in the forthcoming Firefox OS.