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

Thunderbird, Postbox

Add Email

Copy Email Address to Address Book

Add a new email address to the address book.

Compatibility: Thunderbird, Postbox SeaMonkey

Change Text Direction

Change the text direction from left to right or right to left

Change the text direction from left to right or right to left.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Edit Draft

Edit Saved Draft or Message

Edit a message that you have saved as a draft.

Compatibility: Thunderbird, Postbox SeaMonkey

Format

Show the Format Menu

Show the Format Menu

Compatibility: Thunderbird, Postbox SeaMonkey

Get Themes

Get more Thunderbird themes

Open the Themes Manager to change your theme or install a new one. Only works in Firefox 1.5. Firefox 2 users should use the Add-ons Button.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Help

Open help documentation

Open the application help documentation.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Insert

Show Insert Menu

Show Insert Menu

Compatibility: Thunderbird, Postbox SeaMonkey

Languages

Open Language Manager

Open the Language Manager to enable or disable or do any other needed tasks to any of your language packs.

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

Open in Conversation

Open current message in Conversation

Open the current message in a conversation tab

Compatibility: Thunderbird, Postbox SeaMonkey