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

Thunderbird, Postbox

Work Offline

Toggle work offline

Toggle Work Offline and Work Online.

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

about:about

List and open any of the about pages

Lists all the "about" pages so that they can be opened.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

about:config

Open about:config to change settings

Open about:config to change hidden settings that cannot be edited from the Options window.

All Mozilla based programs have settings that can not be changed in the options window. These hidden settings are all reveled in the about:config where anything can be changed or reset.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

HTML Mode

Set if Message is shown as HTML or Plain Text

Set if the message is displayed as HTML, sanitized HTML (all the possible nasties removed) or as plain text.

Compatibility: Thunderbird, Postbox SeaMonkey

Page Colours

Toggle between the default and the site's page colours

Toggles between using the default colors (that tend to be easily readable) and the colors set by the web pages author.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Paste

Paste

Paste text.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Print Preview

Open Print Preview

Open the Print Preview to view a page before printing it.

Paper costs money (and trees) so it is worth checking what the page looks like before you print it. Though it has always amazed me just how much the button gets downloaded.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

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