Windows users can copy and paste images (including screen shots) from the Windows clipboard into HTML mail compose.
Only mark a message as read when it has been open in the preview pane for a configurable number of seconds.
More options for customizing toolbars the way you want them. Enchancements include: a mark button in the main mail window and copy/cut/paste icons in mail compose.
Profile Migration from Netscape 4.x has seen a lot of improvements.
A Palm Sync Address book conduit extension is now available for Thunderbird 0.4.
POP Accounts now support aging on the server.
Thunderbird 0.4 also adds a "Windows" menu in Mac OS X, improves menu bar handling when no windows are open, and fixes a Mac-specific bug that occurred when running it at the same time as Firebird.
Thunderbird is available for Mac OS X, Linux and Windows. It can be downloaded at the Thunderbird home page.