The official repositories for a particular version of Ubuntu are composed to contain the latest versions of software packages as of the date of release of that version of Ubuntu. After the release is made, newer versions of software packages do not get added to the repositories, with the exception of security fixes. So, for example, the latest version of Firefox during Ubuntu 8.04 Hardy release was 3.0, so the repositories will contain 3.0, even though Firefox 3.5 was released afterwards. Furthermore, even the security patches are usually several days to a week or more behind the official Mozilla releases, due to the time it takes to test and package the software.

viaSourceForge.net: ubuntuzilla.