These are a ready make selection of some of the most popular buttons from the Custom Toolbar Button Maker

Thunderbird, Postbox

Format

Show the Format Menu

Show the Format Menu

Compatibility: Thunderbird, Postbox SeaMonkey

Forward As Attachment

Forward message as an attachment

Forward the current message as an attachment.

Compatibility: Thunderbird, Postbox SeaMonkey

Forward Inline

Forward message as part of the email

Forward the current message as part of the email.

Compatibility: Thunderbird, Postbox SeaMonkey

Get Inbox

Change Focused Folder to the Inbox

Change the currently selected folder to your Inbox.

Compatibility: Thunderbird, Postbox SeaMonkey

Go

Show the Go Menu

Show the Go Menu

Compatibility: Thunderbird, Postbox SeaMonkey

Insert

Show Insert Menu

Show Insert Menu

Compatibility: Thunderbird, Postbox SeaMonkey

Minimize

Minimize current window

Minimize the current window.

I hate popups that don't show the minimize button on the window itself. This has been my solution. Also when I use full screen mode, and then want to minimize the window, this again this comes in useful.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

New Event

Add a new event to the calendar.

Add a new event into the calendar.

Compatibility: Thunderbird, Postbox SeaMonkey

New Task

Add a new task to the calendar.

Add a new task to the calendar.

Compatibility: Thunderbird, Postbox SeaMonkey

Options Menu

Show Options Menu

Show the options menu in the compose mail window.

Compatibility: Thunderbird, Postbox SeaMonkey