I updated my patch for tabbed message browsing in Thunderbird to work on the trunk, and I built trunk test builds for Windows, Mac, and Linux. For folks who want to try out the feature but don't want to ride the cutting edge, I also built 1.5 branch builds for Windows, Mac, and Linux.
If you haven't tried message tabs before, using them is simple. Just right-click a message header and select "Open Message in New Tab" from the context menu. You can also make the Enter key open messages in tabs by setting the Preferences -> Advanced -> Open new messages in preference to A new message tab.
Tabs work much like they do in Firefox: you can close them with Ctrl/Cmd-w, reorder them with drag-and-drop, etc. Next time you want to save some messages to read a little later, or you want to work with several messages at once, but they aren't all consecutive in the same folder, try opening the messages in tabs. I reckon you'll find them easier to use than windows.