Xul: Creating toolbar buttons
Mozillazine has a great article on creating toolbar buttons. One thing they sort of go into is toolbar palettes -- here are the toolbar palettes I've run across:
MailToolbarPalette: the toolbar in Thunderbird's main window
MsgComposeToolbarPalette: the toolbar for a Thunderbird compose window
AddressBookToolbarPalette: the toolbar for the address book toolbar palette, but if you're trying to write an overlay for the address book, you'll need to modify your
contents.rdffile to add an overlay for the address book .xul file.