Firefox 25 has been made into Ubuntu 13.10 Saucy, Ubuntu 13.04 Raring, Ubuntu 12.10 Quantal, Ubuntu 12.04 and their derivatives.
1. First install Synaptic Package Manager from Ubuntu Software Center.
2. Open Synaptic and Click Reload button to update package lists.
3. Search firefox and choose it from the list, click “Mark for Upgrade” in its context menu.
4. Finally click Apply button.
@Athi, Canonical has dropped support for Ubuntu 13.04 Raring. So the packages might be removed (not sure) from the server. You’d better to upgrade to Ubuntu 14.04 or Ubuntu 12.04 LTS (Long-Term Support).