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

Thunderbird, Postbox

Delete

Delete Selected Text

Delete the currently selected item(s) or text.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Passwords

Open Password Manager

Opens the password manager to view saved passwords

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Menu

All menus

Show all the menus in a single menu.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Redo

Redo Last Change

Redo the late change.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Zoom In

Enlarge Text Size

Enlarge the text size on sites that makes the text so small you can't read it.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

about:plugins

Open about:plugins to view installed plugins

Open about:plugins to view installed plug-ins.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Reset Zoom

Reset Text Size

Reset Text Size to the default setting.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Save

Save document as

Save the currently open document, or image to a file.

Saving a page or email for later viewing or to send to a friend is a much needed functionality, so much so that there really should be a toolbar button for it.

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

Edit as New

Edit Message as New

Edit an email.

Compatibility: Thunderbird, Postbox SeaMonkey