How to add a custom toolbar button

Save the following settings to a text file, and import it via "TU Options > Settings > Import...".

"Notepad":

pref("extensions.tabutils.button.button_Notepad", true);
pref("extensions.tabutils.button.button_Notepad.label", "Notepad");
pref("extensions.tabutils.button.button_Notepad.path", "C:\\WINDOWS\\system32\\notepad.exe");
pref("extensions.tabutils.button.button_Notepad.image", "moz-icon:file://C:/WINDOWS/system32/notepad.exe?size=16");
pref("extensions.tabutils.button.button_Notepad.parent", "toolbar-menubar");
pref("extensions.tabutils.button.button_Notepad.oncommand", "var file = Cc['@mozilla.org/file/local;1'].createInstance(Ci.nsILocalFile);file.initWithPath(this.getAttribute('path'));if (file.exists()) file.launch();");
pref("extensions.tabutils.button.button_Notepad.tooltiptext", "Notepad");