These are a ready make selection of some of the most popular buttons from the Custom Toolbar Button Maker
Thunderbird, Postbox
Message
Show the Message Menu
Compatibility:
Thunderbird, Postbox
SeaMonkey
Message Pane
Show/Hide the Message Pane.
Compatibility:
Thunderbird, Postbox
SeaMonkey
New Event
Add a new event into the calendar.
Compatibility:
Thunderbird, Postbox
SeaMonkey
New Task
Add a new task to the calendar.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Next Message
Get the next message. Unlink the default Thunderbird buttons this just go to the next in the list, or if shift is pressed, the next unread.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Not Junk
Mark the selected messages as Not Junk.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Open in Conversation
Open the current message in a conversation tab
Compatibility:
Thunderbird, Postbox
SeaMonkey
Options Menu
Show the options menu in the compose mail window.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Paste As Quotation
Paste text so that it appears as a quoted message.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Plain Text Paste
Paste text into Thunderbird without any formatting.
Compatibility:
Thunderbird, Postbox
SeaMonkey