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

Thunderbird, Postbox

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

Not Junk

Mark the selected messages as not junk

Mark the selected messages as Not Junk.

Compatibility: Thunderbird, Postbox SeaMonkey

Plain Text Paste

Paste Text With No Formatting

Paste text into Thunderbird without any formatting.

Compatibility: Thunderbird, Postbox SeaMonkey

Purge

Compact Folder, and Delete Junk

Delete all junk emails then compact all folders. Quick way to do both at the same time.

Compatibility: Thunderbird, Postbox SeaMonkey

Return Receipt

Toggle Return Receipt on/off

Toggle the sending of a return receipt on and off.

Compatibility: Thunderbird, Postbox SeaMonkey

Rewrap

Rewrap

Rewraps the message text

Compatibility: Thunderbird, Postbox SeaMonkey

Save Attachments

Save All Attachments From Message

Save Attachments from an message.

Compatibility: Thunderbird, Postbox SeaMonkey

Search Addresses

Open Addresses Search

Open the advanced address search window.

Compatibility: Thunderbird, Postbox SeaMonkey

Send & Receive

Send and receive messages

Send all messages in the outbox and get all new messages.

Compatibility: Thunderbird, Postbox SeaMonkey