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

Install Format

Download Format

Compatibility: Thunderbird, Postbox SeaMonkey

Forward As Attachment

Forward the current message as an attachment.

Install Forward As Attachment

Download Forward As Attachment

Compatibility: Thunderbird, Postbox SeaMonkey

Forward Inline

Forward the current message as part of the email.

Install Forward Inline

Download Forward Inline

Compatibility: Thunderbird, Postbox SeaMonkey

Get Inbox

Change the currently selected folder to your Inbox.

Install Get Inbox

Download Get Inbox

Compatibility: Thunderbird, Postbox SeaMonkey

Go

Show the Go Menu

Install Go

Download Go

Compatibility: Thunderbird, Postbox SeaMonkey

Insert

Show Insert Menu

Install Insert

Download Insert

Compatibility: Thunderbird, Postbox SeaMonkey

Minimize

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.

Install Minimize

Download Minimize

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

New Event

Add a new event into the calendar.

Install New Event

Download New Event

Compatibility: Thunderbird, Postbox SeaMonkey

New Task

Add a new task to the calendar.

Install New Task

Download New Task

Compatibility: Thunderbird, Postbox SeaMonkey

Options Menu

Show the options menu in the compose mail window.

Install Options Menu

Download Options Menu

Compatibility: Thunderbird, Postbox SeaMonkey