I filed a fair number of bugs on Thunderbird last weekend calling out some of the bits of interface that were causing me aesthetic stress. I was on a loaner laptop without photoshop and my other tools. So while I could point out the ugly bits, I wasn't able suggest how specifically those bits needed to change.
Here's another quick photoshop hack-job to try to remedy that. This is the primary Thunderbird window with the alternative "vertical view" -- my primary view, and the view that I think ought to be the default given how many widescreen laptops and desktops there are these days.
The first image is what Thunderbird looks like for me with no customizations except for switching from the standard view to the vertical view. The image below that is my mock up of what I think would make a much more pleasing interface.
My primary goals in this redesign were 1) to clean up the visual clutter and disorganization. 2) to increase the relative size of the message viewing area.
This isn't an entirely new theme -- the iconography and the buttons and other widgets (with two exception) are all the same. It's just a bunch of visual alignment cleanup and hiding away a few rarely used bits to try to free up more space for reading email.
You can click on the image to get the full-sized view. Maybe open it in another tab and continue reading here :-)

I've intentionally sized the window a bit smaller than my smallest laptop resolution so that it would be easier to view the images and to show that this view can scale all the way down to screens with 1024 x 768 resolution. A larger window looks even better, IMO.
What I would change (in no particular oder):
1. Drop Aero Glass on the toolbars. It doesn't add any value and actually makes it harder to read and mouse target what ever is on those Glass toolbars. Aero Glass is nice, and it's got its place. I think Firefox's use of glass behind the tabs is actually OK. But it doesn't work everywhere and shouldn't be used just "because we can".
2. Hide the menubar and hide the statusbar. This is where Windows is going. Users know or will learn that Alt pops the menubar back into view. If a user absolutely cannot deal with it being hidden, then provide an option to show it by default just like other Windows apps. The statusbar should become transient like Firefox's statusbar.
3. Get all of the various toolbars and rows of text aligned and using the same colors and fonts. There's just no good excuse for having toolbars stacked at different heights and text or labels that don't line up and random colors that don't actually signal anything to the user.
4. Move rarely used features out of the primary UI. Users don't need toggles for every UI option in primary UI. Showing and hiding the "Filter" bar is not something users need to change every few minutes. Most users don't need to download attachments many times a day. The online/offline toggle isn't something that deserves an entire toolbar. These things can live in less conspicuous places.
5. Remove redundancy. Two search fields, even if one is technically "search" and one is technically "filter" make for confusing UI. Users don't need to see the time and date of the message in the Thread pane and in the Message pane. The unread message count is available in the Server/Folder pane. It doesn't need to also live in a toolbar.
6. Avoid strange and exotic widgets. The scroller thing for the Server/Folder pane is un-obvious and optimizes speed (single click) at the expense of clarity (which view is next in the scroll order??)
7. Maximize the viewing area for common views so users have to scroll less. The Message pane should not be mostly UI. It should be mostly message. The message headers should be scroll out of view to put more message in front of the user. The key pieces of information here, the Subject and the From fields are easily glanced over in the Thread pane if needed.
8. Primary toolbar buttons don't actually need to take up an inch of vertical real estate. Move those to the titlebar like modern Windows apps and kill the big toolbar.
With these changes, Thunderbird not only looks great, but we go from having only 25% of the app's window devoted to the email message you're reading, to almost 45%. I think that's a pretty big win.
I believe I've filed or there are already on file bugs for pretty much everything covered here.
I'm not claiming that this is the ultimate UI for Thunderbird, but I think this would be a dramatic improvement from the current look without an entire re-write or a new set of artwork. I'd be thrilled to return to a Thunderbird that was as well ordered as this mock up.
What do you all think? Have I missed any major issues? Have I hidden your "I can't live without it" features? Would you use a Thunderbird that looked like this?
update: I want to make it clear that I'm not shooting for a "compact" or "minimal" UI here. I'm trying to help the user focus on the things that matter most while making the whole thing more visually appealing. By removing and re-ordering things, Thunderbird can show more of what matters (email!) and less of what doesn't. And remember that the mock-up is at 1024 x 768 so it looks much more spacious and inviting when the window is sized larger for all of us with modern screens.