Ubuntu 12.04.4 features a new version of the Ubuntu kernel, version 3.11, and matching X.org stack, aimed at providing existing LTS users with improved support for ‘a wider variety of hardware’.
While a core part of this release, the updated ‘hardware enablement stack‘ won’t roll out to everyone. Only installations made using Ubuntu 12.04.2 or 12.04.3-based images will automatically receive the updates.
Those who installed using previous versions can choose to opt in to this HES update using the following command:
sudo apt-get install linux-generic-lts-saucy xserver-xorg-lts-saucy
Other changes include new versions of Firefox & Thunderbird and the removal of the Ubuntu One Music Store plugin from Rhythmbox.
Support for Ubuntu 12.04 LTS stretches into 2017. The next LTS release, Ubuntu 14.04, is due this April.