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

Thunderbird, Postbox

Delete Junk

Delete Junk Messages

Empty the junk email folder of all junk email.

Compatibility: Thunderbird, Postbox SeaMonkey

Find

Find Text

Open the find bar to find a word in the current page.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Redo

Redo Last Change

Redo the late change.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Toggle menu bar

Toggle the menu bar

Toggle the menu bar. Don't put this button on the menu bar!

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Copy

Copy Selected Text

Copy selected text.

Compatibility: Pale Moon 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

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

Page Fonts

Toggle between the default and the sites fonts

Toggle between the use of the fonts used by the site and those that are set as the default.

Compatibility: Pale Moon Thunderbird, Postbox SeaMonkey

Options

Open options

Opens the Options window so you can change settings easily.

Firefox, Thunderbird and Sunbird all have a huge number of settings. Often you will find a need to change one. About to visit your banking site, so you want pop ups enabled, or what ever your current need might be. The button brings the option window to you quicker.

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