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

Restart the current application after installing a new extension, theme or plug-in.
There is a number of reasons you may want to restart the program your using. Though having installed an extension is the most common. This button take all the hassle and work out of it.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
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
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
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
Work Offline

Toggle Work Offline and Work Online.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
about:about

Lists all the "about" pages so that they can be opened.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey
Source

View the page or message source code.
Compatibility:
Pale Moon
Thunderbird, Postbox
SeaMonkey