Ubuntu – Extended support for Ubuntu 12

The core aspects of Ubuntu get full support for the full period. That includes kernels, pre-installed applications. You should be fine with Firefox updates. It's the XFCE stuff that will start to lag behind.

The repositories for Xubuntu and Ubuntu are the same — it's just a different selection of packages. If you want full Xubuntu support indefinitely you either need to upgrade in a timely manner or maintain (or pay somebody else to maintain) all the packages you need indefinitely.

The best information I have been able to find about the program is this. It seems to spell out in detail the level of service as well as the response times you can expect for varying levels of severity. Certified hardware is of course better supported.

Hopefully someone with personal experience can provide you with a better answer.