Dev : Extensions : Example Code : Apply button
From MozillaZine Knowledge Base
How do I add an Apply button to my dialog?
<dialog xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul" id="blah" .... buttons="accept,cancel,extra1" ondialogaccept="onAccept();" ondialogextra1="onApply();" buttonlabelextra1="Apply" buttonaccesskeyextra1="A"> <!-- ... --> </dialog>