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

View the page or message source code.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Status Bar

Toggle the status bar from hidden to shown. Hide it to get the most out of your screen space, and show it when it really needed.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Switch Profile...

Switch the Profile that you are currently using.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Tasks In View

Toggle the Task in View panel in Sunbird.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Themes

Open the themes manager to change you theme or install a new one.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Threads with Unread

View only the threads in the current folder that have unread messages.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Toggle menu bar

Toggle the menu bar. Don't put this button on the menu bar!
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Toggle Read

Toggle the Read status of the currently selected message.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Toggle Starred

Toggle the starred status (look for the star next to the message).
Compatibility:
Thunderbird, Postbox
SeaMonkey
Toggle Tasks

Toggle the task bar between visible and hidden.
Compatibility:
Thunderbird, Postbox
SeaMonkey