Tabby!

Ladies and Gentlemen,
SeaMonkey has tabs now!
Wait, you’ll say, but it already had since the very beginning?
Well, not quite. SeaMonkey had some tabs, but only in the browser and in certain dialogs. Now, you can let your productivity run free in MailNews as well!
Wait again, but I don’t see any tabs there?
Currently, MailNews tabs (mostly) adhere to their browser counterpart’s settings¹, hence the tabstrip is hidden if there’s just one tab. And this first tab is just your default MailNews layout.
Now, let’s do a little carpet ride, nosing under the new hood.
Imagine we’re in your Inbox and we find some new mails by your granddad and your new girl friend. Certainly, you’ll want to read them both, but like other strange people you’re weak in reading two letters at the very same time, hence you open granddad in a background tab and start business with the serious stuff.
I do what? How?
Everywhere, where you could open a new message window in older versions of SeaMonkey, you can now open the message in a new tab. For instance, right-click an entry in the thread pane and choose “Open in New Tab”. Or just do a middleclick, if you have your browser tabs configured to open new tabs on that. If you also have chosen to open new browser tabs in the background, new MailNews tabs will adhere to it as well, else the new tab will come up in front.
The most visible change with such “message tabs” is their layout, which basically resembles the standalone message window. No folderpane, no threadpane, just the pure message for your enjoyment. 😉
Hm, your girl friend is relating to something she wrote a while ago. That mail should still be in the Inbox, you say? Just hit Shift-F8 to make the threadpane appear again, type her name into the search field, and open the old mail via the thread pane context menu! F8 (toggle messagepane) and F9 (toggle folderpane) will work as they used to, as does the “View → Layout” menu or clicking the splitter grippies. In fact, the only difference between “message tabs” and normal “folder tabs” is the initial ad-hoc layout change. And of course, switching between tabs will remember your last layout state.
Anyway, that girl deserves a reply, but first let’s print the picture she sent. So, wait, huh, the printer button is gone from the main window and the preference panel hasn’t it anymore either?
Introducing customizable toolbars:
If you right-click toolbars in MailNews, you can open a customization dialog now. Drag the printer button from the palette back onto the toolbar and that’s it!
For some obscure reason, the Compose button is still on the main toolbar by default, but now, like many of his colleagues, it has a dropdown menu as well, so you can choose between the HTML and the plaintext editor.
Okay, printing done, reply written, what’s next? Ah, yes, granddad’s still waiting to be read in tab one. And he has news to tell — he’s found the SeaMonkey blog RSS feed! Let’s just add that feed URL to your MailNews feed account and move on.
Good joke, but MailNews is just, huh, mail and newsgroups …
Ah, wrong again, dear reader!
From the main menu, choose “Edit → MailNews Account Settings” and click the “Add Account” button. Select “Blogs & News Feeds” as the account type and choose a suitable account name. Back in the main window, you’ll find a new account with the standard RSS symbol. Open the subscribe dialog from the new account’s context menu and add <http://weblogs.mozillazine.org/seamonkey/index.rdf> as your new feed.
Done!
Mmh, the Great Old One relates to a newsgroup thread, thus we’re in need of a second folder tab: right-click the newsgroup and choose “Open in New Tab” … And given that this is one of your most read newsgroups, it’s time to drag it into the pole position of that account.
Although … As you remember (I hope!), there was another post in another newsgroup, which would answer granddad’s question profoundly. Thus, we clone the current folder tab by hitting Ctrl+T (or “File → New → Duplicate Tab” or doubleclicking the folder, if browser.tabs.opentabfor.doubleclick is set to true).
But Ctrl+T is Get New Messages!
Sorry, but no. For consistency’s sake, we made Ctrl+T open new tabs both in browser and MailNews. Downloading new mail is now Ctrl+D/Ctrl+Shift+D.
Phew, please, just close the window, I’m done…
“This messenger window has 4 tabs open. Do you want to close it and all its tabs?” 😉
So much for now from SeaMonkey MailNews 2.0 land; stay tuned² …

¹ This will change, see bug 514476.
² Thou Shalt Smite Lightning Onto The Unbelievers?