I'm using Ubuntu 12.04 and my Firefox version is 14.0.1. Since because of this version I can't install Firebug. So for upgrading it I searched at the Help->About in the browser window, but there is no update option. Then I searched in the Ubuntu Update Manager, and there are listed very many updates, but I didn't find "Firefox browser update". I'm new to Ubuntu, and I don't know how to work in the terminal so I haven't tried that.
To update just Firefox, you could try the following:
I'd recommend installing the rest of the updates at your earliest convenience.