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

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
Undo

Undo last change/action.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Select All

Select all the text in the current text box, web page, etc.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Add-ons

Opens the Add-ons window to perform tasks on your installed Extensions, Themes, Plug-ins and Language packs.
The Add-on's window allows the installation and management of all your extensions and themes. If you need an update, change a setting, swap themes or enable/disable an add-on the Add-ons' window is the place to go. Being able to access this quickly is a necessity for anyone that uses even a handful of add-ons.
The button also contains a drop down menu that allows you to install add-ons, open the add-ons window to the extension or themes tab, or set which websites can install addons.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Delete Junk

Empty the junk email folder of all junk email.
Compatibility:
Thunderbird, Postbox
SeaMonkey
Empty Trash

Empty the Trash Folder
Compatibility:
Thunderbird, Postbox
SeaMonkey
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
Redo

Redo the late change.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Zoom Text Only

Toggle between text and page zoom.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Delete

Delete the currently selected item(s) or text.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey