As with Firefox, there are Windows, Mac, and Linux versions of Thunderbird available, and you get a choice of 43 languages from Afrikaans to Ukrainian. I tested on Windows 7, Mac OS X Snow Leopard, and Ubuntu Linux. The installer is a slight 8.4MB on the PC (18.6MB for Mac, and 10.3MB for Linux). Once you've run the standard installation, you're left with a blank application surmounted by a dialog asking you simply for your name, e-mail address, and password. Mozilla stores configuration data for common mail services, so in many cases that's all you'll need to enter those to get up and running. Version 2 asked you about POP, IMAP, and SMTP, regardless of whether the account was with something like Gmail or a lesser-known host.
When I entered the username and password for a Hotmail account in Thunderbird 3, the next dialog had all the server fields automatically filled in, and I could just hit Create Account to add the account to the mail app. After this, a dialog asked whether I wanted Thunderbird to be my default reader for newsgroups as well as e-mail and feeds. I could also indicate whether mail would be accessible to Windows Search. But for some reason, I couldn't actually receive mail in the account.
Trying to set up a Mail.com account failed too, but when I tried creating an entry for AOL, the software did better. And it warned me that the AOL servers don't use encryption, which it considered a no-no. The dark red dialog even included a checkbox saying "I understand the risks" that I had to check before the account would be allowed. A final account using a friend's server seemed to set up correctly, but then wouldn't receive or send e-mail either. An AOL account worked swimmingly with auto-setup, however.
Once I got an account working in the software, a super-simple display appeared: a standard window with menu choices atop, just four toolbar buttons under that, and the standard left-side panel showing accounts' mailboxes and folders. Like Windows Live Mail, Thunderbird lets you access multiple e-mail accounts from this left sidebar. It defaults to "Smart Folders," which let you combine inboxes while still showing the separate account entries below the combined inbox. You have a choice of three views—Classic, Wide, and Vertical—but the choices aren't as fluid as in Outlook, which also offers Autopreview and, in Office 2010, a sophisticated conversation view.
In the main window area, the interface is truly hand-holding. Only two choices show up under the E-mail section at the top: Read messages, and Write a new message. Sections below this with similarly few options let you view account settings or add a new account, and Advanced options let you search messages and manage message filters.
The big news in Thunderbird 3's interface is tabs. Because Thunderbird is actually built on top of the same code base as Firefox, its tabs look and act pretty much identically to the browser's, although I wish they had the X's to close them as Firefox does.
If you double-click an e-mail's entry, it opens in a new tab. You can also right-click any left-panel folder or account and choose Open in New Tab. Yahoo Mail has had tabs since 2007, and I think it makes sense in a browser-based interface, but I prefer a standalone client to pop out a new window when you double-click a message entry. Conversely (or perversely), when you start a new outgoing e-mail message, it opens in a new window—here I'd prefer a new tab, the way Yahoo Mail does it. And one final kvetch: You can't drag tabs out to a new window as you can in Firefox.