Approximately two months after the release of the 3rd Beta in the series of Thunderbird 3 releases, the Mozilla Foundation has announced that it reached another milestone in the development of its email client: Thunderbird 3 Beta 4 has been made available for download. The software, which is available for download to Windows, Mac OS X and Linux users, comes with a long list of changes and improvements.

But first, here is what Mozilla had to say: “We're happy to announce the release of Thunderbird 3 Beta 4, now available for download. Thunderbird 3 Beta 4 is available for testers, extension developers, and other people who are curious to follow the development of the next release of Thunderbird. This milestone is focused on testing the core functionality of the new features and platform changes that will be included in Thunderbird 3. Thunderbird 3 Beta 4 includes several new features like Tabbed email and Advanced Search tools that build on the current Thunderbird 2.”

And now, here are the changes and improvements you can expect to get with Thunderbird 3 Beta 4, in brief:

• - New Search with Advanced Filtering Tools
• - New Global Search Field with Autocomplete
• - New Mail Account Setup Wizard
• - Redesigned Mail Toolbar
• - Tabbed Email Messages
• - Smart Folders
• - New Message Summary View
• - Column Headings
• - Message Archive
• - Activity Manager
• - New Add-ons Manager
• - Improved Address Book
• Improved Gmail Integration
• Integrated with Vista search results (Windows version only)
• Thunderbird 3 can import from Mail.app, can read your OS X address book, and can use Growl for new mail alerts (Mac OS X version only)
• IMAP Folder Synchronization

If you would like to get Thunderbird 3 Beta 4 for Windows, Mac OS X or Linux, a download location is available here.
Do not forget to check out the Release Notes as they offer a more detailed view of what’s new in Thunderbird 3 Beta 4 (more detailed than the summary we’ve presented above).