unfortunately this is a regression in firefox 12 on mac os x that has been reported here in the forums quite few times. there are already some bugs filed for the issue & hopefully it will be resolved in one of the upcoming releases (although judging from the bugs' stati there isn't much progress yet). https://bugzilla.mozilla.org/show_bug.cgi?id=264030 https://bugzilla.mozilla.org/show_bug.cgi?id=752149 (please don't post support questions in the bugs)