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

Thunderbird, Postbox

Reset Zoom

Reset Text Size

Reset Text Size to the default setting.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Return Receipt

Toggle Return Receipt on/off

Toggle the sending of a return receipt on and off.

Compatibility: 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

Select All

Select All Text

Select all the text in the current text box, web page, etc.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Tools Menu

Show the tools menu

Show the tools menu.

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

Zoom Out

Reduce Text Size

Reduce the text size so it is not so big.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Archive Thread

Send the Selected Thread to the Archive

Send the Selected Thread to the Archive

Compatibility: 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

Reload Proxy

Reload Proxy

Reload the currently in use proxy.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey