Be careful with old versions!
These versions are displayed for reference and testing purposes. You should always use the latest version of an add-on.
This includes vast improvements for the styling of add-ons manager, some forgotten updates for about: pages in addition to the routine updates for the changes done to match default themes.
The most notable changes compared to 2.0 Beta 2 are support for customizable mail compose toolbars and consistent height of mail tabs.
See also http://home.kairo.at/blog/2009-03/earlyblue_and_lcarstrek_for_seamonkey_2