Fixed :-moz-placeholder deprecated, removed in Firefox 24

Text inputs in XenForo 1.2 use the ":-moz-placeholder" psuedo-class, which was deprecated in FF19 in favour of "::-moz-placeholder" and appears to have been removed entirely in Firefox 24 (I can't find a bugzilla report to back this up, but it does appear to be the case when using the latest Aurora).

This means that the "formElementPrompt" property doesn't function correctly on the latest version of Firefox.