Canonical announced earlier today, June 15th, in a security notice, that an update for Unity 2D is now available for its Ubuntu 12.04 LTS (Precise Pangolin), Ubuntu 11.10 (Oneiric Ocelot) and Ubuntu 11.04 (Natty Narwhal) operating systems.
The update fixes a bug in Unity 2D that was exposed by the Mozilla Firefox 13.0 update issued by Canonical last week, which resulted in Firefox being unable to open popup menus.
The bug can be fixed if you upgrade your system(s) to version 5.12.0-0ubuntu1.1 for Ubuntu 12.04 LTS, 4.12.0-0ubuntu1.2 for Ubuntu 11.10, and 126.96.36.199-0ubuntu1.1 for Ubuntu 11.04. To apply the update, run the Update Manager application or follow these instructions: https://wiki.ubuntu.com/Security/Upgrades.
Don't forget to restart your Unity 2D session or reboot your computer after the update! For more details you can check out Launchpad bug.