In a post couple weeks ago, we did mention about the numbering change for the next Thunderbird release (Miramar) going from 3.3a to 5.0b. However, we have received several questions related to the whereabouts of Thunderbird 4. Simply put there will NOT be a Thunderbird 4.0 version. The reason for this is because Mozilla wants the Firefox, SeaMonkey and Thunderbird to be on the same development cycle which also includes the same version numbering.
Currently Firefox is on version 4.0.*, so why not make Miramar Thunderbird 4.0? A couple reasons, Miramar is not ready for release yet (estimated release date is July 5th) and Firefox 4 support ends with the release of Firefox 5 on June 21st. Keep in mind Firefox 5 is on an accelerated development cycle which means its release date is two weeks prior to Thunderbird 5. Firefox 6 and Thunderbird 6 should follow the same development/release cycle with release on August 16th.