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

Delete all junk emails then compact all folders. Quick way to do both at the same time.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Remote Content

Opens the window to set exceptions for the loading of remote content
Compatibility:
Thunderbird, Postbox
SeaMonkey
Reply to Sender Only

Reply only to the sender of the email.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Run Application

Runs an external application
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Save Attachments

Save Attachments from an message.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Search Addresses

Open the advanced address search window.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Search Messages

Open the advanced message search window to find an email.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Send & Receive

Send all messages in the outbox and get all new messages.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Send Without Save

Send message with out saving a copy to the sent folder. If you want this as the permanent behaviour, there is an option in account settings under 'Copies & Folders'
Compatibility:
Thunderbird, Postbox
SeaMonkey
Skip Trash

Delete a message with out sending it to the Trash. This will completely remove the message so that it can not be recovered.
Compatibility:
Thunderbird, Postbox
SeaMonkey