In Ubuntu 10.04, it is uses Mozilla Firefox 3.5 by default, and you might need to wait(sometimes it is long times) for the Ubuntu team to approved the Firefox 3.6.6 to updates automatically.
To let yourself to get the latest Firefox installed manually, just follow the step as below.
1. Open the Terminal.
2. Type “sudo add-apt-repository ppa:ubuntu-mozilla-security/ppa”
3. Enter the prompt administrator password and make sure your Firefox is closed.
4. Then Type “sudo apt-get update && sudo apt-get upgrade” from the Terminal again.
It will update your Update list from Ubuntu and allow you to install Ubuntu 3.6.6. automatically.