This seems to be a nuisance on a scale worth warning others about: Thunderbird 3 taketh away, and you may have to jump through hoops to get back the lost functionality.
First, the headers in each message are now shown no matter what your preference. In order to get back the screen space you've just lost, you need to find and install an add-in. Seriously?! Yes. It's very lovely, I'm sure, that the junk mail, reply and delete buttons are now displayed in that space. Or not.
Second, the count of the total number of messages in each folder is now gone. In order to get it back, you have to find, and (the version number) and install an add-on. Seriously?! Yes.
Third, the words "Local folders" get appended to every folder name until you install the above-mentioned add-on. I really could use that screen space, thankyouverymuch.
And, last on the list for this moment, the All or Unread messages control seems to have vanished. I haven't found the way (setting or add-on) to bring it back yet.
Come on, team . . . Thunderbird 2 had a workable interface. I only upgraded to 3 in hopes that it would go faster! Please don't take away capabilities as the price of staying up-to-date with current releases and enjoying whatever new features you wish to offer us!
And don't get me started on the number of hours it took Tbird 3 to index my existing e-mail folders.
Overall . . . my recommendation is that you upgrade with your eyes open. Look at the above list, and if none of these things will be a thorn in your flesh, then go for it.