It seems that Firefox 15 changed to the same functionality that Chrome and other browsers use. Which means that the placeholder text disappears only if you start typing. Not when you click the area.
http://userstyles.org/styles/72079/plac ... r-on-click
Could you add the CSS from there in phpbb also?
These requests for comments/change have lead to an implemented feature that has been successfully merged into the 3.1/Ascraeus branch. Everything listed in this forum will be available in phpBB 3.1.