The problem can be corrected by updating your system to the following package versions: Ubuntu 12.04 LTS: gtk3-engines-oxygen 1.0.2-0ubuntu3 In general, a standard system update will make all the necessary changes.
https://ubuntu.com/security/notices/USN-2936-2
https://ubuntu.com/security/notices/USN-2936-1
https://bugs.launchpad.net/ubuntu/+source/firefox/+bug/1575781
https://launchpad.net/ubuntu/+source/oxygen-gtk3/1.0.2-0ubuntu3
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