The team behind the browser Firefox, has released the first 18 months, a major release e-mail client. Being free and open source, Thunderbird has been prepared in the offices of Environment under Windows, Mac OS X and Linux.

Interestingly, Thunderbird skipped numbering 4.x, in order to comply with the current lineup Firefox 5.
Mozilla Thunderbird 5.0 is characterized by a high-speed downloading, user interface and reorganized with movable tabs, highly informative, intelligent setup wizard email accounts (just enter the email address and password – all other settings will be adjusted automatically), the extension manager functionality.
Mozilla promises that Thunderbird is similar to Firefox went into the process of rapid development: new releases will appear every three months.
via