According to the W3C, the <i>"User agent's default style sheet may override this value. For example, user agents typically do not allow selection of the contents of a BUTTON element.</i> [http://www.w3.org/TR/1999/WD-css3-userint-19990916#user-select] As the -moz-user-select was designed to mimic the user-select property, the same applies.