Thunderbird 3.0 was released at the end of 2009. This was before Mozilla adopted a Chrome-like versioning strategy that led to rapid number inflation for both Thunderbird and Firefox. (1.0 was released in 2004; 2.0 was released in 2007; 3.0 was released in 2009; 3.1 in 2010; 5 (there was no 4) in 2011 commencing rapid versioning. See http://en.wikipedia.org/wiki/History_of_Mozilla_Thunderbird#... )

3.0 was a radical departure from and regression in speed, stability, and usability relative to the 1.x and 2.x series.