The first test builds of Thunderbird 3.0b4 are available here (hint for windows users they are in the unsigned directory.)
Along many bug fixes and improvements the major new feature is the way Thunderbird handles search and search results :
For a full list of what's new, please take a look at https://developer.mozilla.org/en/Thunderbird_3_for_users
We need our users to give it a try. Any bugs found with these builds should be marked blocking bug 515237.
If Thunderbird crashes, make sure to send the crash report and make sure to add a comment (as those comments really help).
If you could monitor Thunderbird's memory usage and report if it looks ok or not. For report please make sure to monitor Real+Virtual+ ... not just real memory.
OpenSuse users can fetch it from the mozilla:beta repository.