These are a ready make selection of some of the most popular buttons from the Custom Toolbar Button Maker
Thunderbird, Postbox
Go
Show the Go Menu
Compatibility:
Thunderbird, Postbox
SeaMonkey
Ignored Threads
View all the threads in the current folder that are set as ignored.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Insert
Show Insert Menu
Compatibility:
Thunderbird, Postbox
SeaMonkey
Message
Show the Message Menu
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
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
Read
Open the currently selected message in Thunderbird.
Compatibility:
Thunderbird, Postbox
SeaMonkey