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

Thunderbird, Postbox

Delete Tasks

Delete Selected Tasks

Delete a task.

Compatibility: Thunderbird, Postbox SeaMonkey

Toggle Proxy

Toggle The Proxy On & Off

Toggles the Proxy between on and off for quick changing of networks.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Account Settings

Open account settings

Open Account Settings in Thunderbird.

Compatibility: Thunderbird, Postbox SeaMonkey

Edit Menu

Show the edit menu

Show the edit menu.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Extensions

Open Extension Manager

Open the Extensions Manager to install, uninstall or do any other needed tasks to any of your extensions.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

New Task

Add a new task to the calendar.

Add a new task to the calendar.

Compatibility: Thunderbird, Postbox SeaMonkey

Toggle menu bar

Toggle the menu bar

Toggle the menu bar. Don't put this button on the menu bar!

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Cookie Exceptions

Open the Cookie Exceptions dialog

Open the cookie exceptions dialog.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Cookie Manager

Opens the cookie manager for the current web page.

Opens the cookie manager for the current web page. On middle click opens in a new tab.

Compatibility: Pale Moon 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