Mozilla released a major update Tuesday to the beta build of its desktop e-mail client Thunderbird, debuting a new search feature, "smart" folders, an activity manager for monitoring communication with your mail server, and better Gmail integration among the list of improvements. Thunderbird 3 beta 4 is available for Windows, Mac, and Linux.

Search results in the new beta open in their open tab, and are filterable. (Credit: Screenshot by Seth Rosenblatt/CNET)

The smart folder view condenses your various in-boxes into one gestalt in-box, but it's easy to switch back to the traditional view if you don't like it. In the folders pane, use the left and right arrows at the top right to change your folder view until you're looking at the one you prefer. Thunderbird will remember your last setting, so when you restart it won't jump back to the smart folder setting. If you have multiple e-mail accounts, though, this should make managing your e-mail stream much easier.

Apparently, there was a lot of code that was overhauled in this fourth beta and that might explain why my default button bar and toolbar settings reverted to normal. It might also have been because of the introduction of the new search bar, which defaults to a significant chunk of window space. The global search bar features auto-complete as well as more robust filtering tools for narrowing down a broad search quickly.

Thunderbird 3 beta 4 introduces tighter folder integration for Gmail users. (Credit: Screenshot by Seth Rosenblatt/CNET)

I found the new search to work well, opening the results in a new tab, although occasionally narrowing down results did not produce the desired result of fewer e-mails in the results window. Searching by tags also appeared spotty at times. Searches do produce a nifty chart, accessible by clicking on the blue bar graph at the top of the search results, that's good for numbers junkies who want to see how often that particular search term shows up.

Windows users should also find that Thunderbird results will now appear when search in Windows Vista and Windows 7, while Mac users will find Growl notification support for new e-mails, integration with the Mac OS X address book, and support for Mail.app. Beta 4 doesn't support legacy versions of Windows prior to Windows 2000 or Mac OS X 10.3 and older.

Thunderbird 3 beta 4 supports the nightly builds of the calendaring plug-in Lightning, but many other add-ons are likely to break until their publishers release compatibility updates. The full list of Thunder 3 beta 4 changes can be read here.