Unlike Firefox, Mozilla Thunderbird has never been a particularly successfully project.
Both Firefox and Thunderbird were split out from the Mozilla Suite (which was based on the monolithic Netscape Navigator codebase) at the same time. Firefox picked up momentum, Thunderbird didn’t.
The reasons why one project has flourished and the other floundered are many and varied.
In September of 2007, Mozilla tried to kick start Thunderbird by spinning it off into its own division. Some four months later they apparently now have some direction and plan. David Ascher who leads the Mozilla Thunderbird group now posted a lengthy discussion on what Thunderbird needs to do to prosper. Ascher is proposing a]public milestone build of Thunderbird 3 in 2008 to get the ball moving.
The reasons why people don’t choose to use Thunderbird are varied,
but two primary reasons appear to be: the lack of a built-in calendar
integration (compared to Outlook for example), or a search experience
that doesn’t match that offered by competitors (gmail and Mail.app for
From my point of view the Calendar issue is long standing, well known and a major issue. Mozilla’s Sunbird effort is supposed to be a stand alone open source calendar that could be used. There is a Mozilla Lightning effort that was originally intended to be Sunbird implementation for Thunderbird – but it has been under development for so long it makes you wonder how actively it’s actually being developed. It will be interesting to see if Ascher can overcome years of Netscape Mail baggage and Thunderbird mis-steps and get this project on track.