Mozilla Thunderbird is a free and open-source cross-platform email client, news client, RSS and chat client developed by the Mozilla Foundation. The project strategy was modeled after that of the Mozilla Firefox web browser. It is installed by default on Ubuntu desktop systems.
On December 7, 2004, version 1.0 was released, and received more than 500,000 downloads in its first three days of release, and 1,000,000 in 10 days.
On July 6, 2012, Mozilla announced the company was dropping the priority of Thunderbird development because the continuous effort to extend Thunderbird's feature set was mostly fruitless. The new development model shifted to Mozilla offering only "Extended Support Releases", which deliver security and maintenance updates, while allowing the community to take over the development of new features
On December 1, 2015, Mozilla Executive Chair Mitchell Baker announced in a company-wide memo that Thunderbird development needs to be uncoupled from Firefox. She referred to Thunderbird developers spending large efforts responding to changes to Mozilla technologies, while Firefox was paying a tax to support Thunderbird development. She also said that she does not believe Thunderbird has the potential for "industry-wide impact" that Firefox does. At the same time, it was announced that Mozilla Foundation will provide at least a temporary legal and financial home for the Thunderbird project.