Thunderbird binaries are available for i386 Linux, Mac OS X, and Windows. I tested the 2.0 alpha build on Ubuntu Dapper Drake. As with any email client, you should always make a backup of your messages before installing a new version -- especially so on pre-release software like this. Also, be aware that most extensions built for Thunderbird 1.5 will not work in 2.0 preview releases.
I spent the weekend using the Thunderbird 2.0 preview and experienced no crashes or data loss. The app is noticeably speedier; initial startup and message retrieval were both faster, though of course your performance may vary depending on the number and type of mail accounts you are checking. The interface is essentially unchanged, but with a few adjustments worthy of discussion.
New and improved
The changes in this release are best described as subtle. At first glance you might miss it, but there is a new button on the toolbar labeled Tag. This is new feature allowing you to mark messages with user-definable tags. It replaces the older Label feature, although by default it supplies you with a set of tags identical to Thunderbird's old set of labels.
The tag system has three advantages over the old label system. First, you can define as many tags as you want (labels were limited to five). Second, you can apply as many tags as you want to each message (labels were limited to one per message). And third, tags are hot, new, and Web 2.0 buzzword-compliant (labels are not). On the other hand, the labels were tied to the hotkeys 0-5; pressing one of the numbers would automatically display all messages tagged (I mean, labeled) with the corresponding label. No such feature seems to be available yet for tags.
A second new but easy-to-overlook feature is revealed by a tiny pair of left and right arrows at the top of the folder list. These buttons let you cycle through the new Folder Views. Previous versions of Thunderbird could only display one collapsible tree view encompassing all of the mail folders configured for the system. The new release adds three additional views: one displaying only folders with unread messages, one displaying recently accessed folders, and one displaying "favorite" folders. I found this an unexpected, welcome addition. I have Thunderbird set up to access five different mail accounts; even with nested folders in the Local Folders hierarchy, the list is often too long to fit on the screen all at once.
Like Firefox 2.0, the new Thunderbird sports an integrated find-as-you-type search. As you type in the search field of the tool bar, the folder contents pane displays a live-refreshed list of matching messages. By default, the search bar searches message subjects only; the other headers are accessible through the pull-down menu. Find-as-you-type search is limited to the contents of the currently displayed folder, though -- to perform a broader search you have to revert to the Search Messages tool.
Several of the same behind-the-scenes changes that hit Firefox have come to Thunderbird as well. Extensions and Themes are now managed together through one tool called Add-ons, finally synchronizing terminology with the addons.mozilla.org Web site.
In addition to the front-page attractions, Thunderbird provides several improvements to the existing features. The new mail notification area has undergone extensive revision -- this is the pop-up transient window that appears when Thunderbird finds new mail on the server during a routine background mail-check. Junk mail and spam filtering has gone through the usual upgrade.
I saw several references to a new "Conversations" feature in the release notes and on individual blog posts discussing the new release. Most referenced the conversations view in Gmail, which made me curious. Having search through Bugzilla and then used the 2.0 preview release, it appears to me that Conversations are exactly the same as the threaded message view in the existing version of Thunderbird. Perhaps since Gmail calls them conversations, Mozilla must as well, but I hardly think that it qualifies as a new feature.
As with all new releases, there are hundreds of fixed bugs, some of which are more significant than others. I was happy to see several bugs fixed in the Address Book component, though my chief complaint (that LDAP directories are read-only) has yet to receive any TLC.
Empty nest syndrome
Almost as notable as the new features are those that did not make it into this release -- although most are still slated for inclusion in the final 2.0.
One that has received a lot of attention is tabbed messaging. Firefox has had tabs for years, and like many users, I wouldn't browse without them. Likewise, I am looking forward to opening multiple email messages at once in tabs within a single Thunderbird window. It makes so much sense.
Much of the discussion in Bugzilla about this feature is over what should and should not be separated into tabs. Should separate accounts be opened in tabs, or just individual messages? If you open a message in a new tab, then change folders, should the new tab reflect this? Unlike Web browser tabs, folders and email have a hierarchical structure, which makes it difficult to refactor the display without confusing the user. I hope they get it sorted out, though, because even though I don't have all the answers, I can see that there is some utility in this concept just waiting to be let out.
Of lesser interest, perhaps, are folder summary pop-ups: tooltip-style summaries of the unread message content in folders. For those of us who deal with a lot of mailing list traffic, this will be a useful addition, particularly when coupled with the new Unread Folders view.
I hope these and the other postponed features will make it into the next beta of Thunderbird and eventually into the official release. As of now, the 2.0 preview is to the Firefox 2.0 preview as Thunderbird is to Firefox: less glitz, less hoopla, but a solid, stable product that you can rely on every day.