Mozilla's released a few weeks ago version 3 beta 3 of it Email Client, Thunderbird. This beta release still full of bugs and should not be used on a production box or as used as your principal email client. See the list of bugs in Thunderbird 3 beta 3 before going ahead to install. Anyway, this installation guide does not replace Thunderbird 2.0.0, if you have it installed, instead you'll be able to test Thunderbird 3 beta 3 alongside Thunderbird 2.