Para atualizar apenas o Firefox por conta própria:
sudo apt-get dist-upgrade firefox
ou
sudo apt-get upgrade firefox
de man apt-get
upgrade
upgrade is used to install the newest versions of all packages
currently installed on the system from the sources enumerated in
/etc/apt/sources.list. Packages currently installed with new versions
available are retrieved and upgraded; under no circumstances are
currently installed packages removed, or packages not already
installed retrieved and installed. New versions of currently
installed packages that cannot be upgraded without changing the
install status of another package will be left at their current
version. An update must be performed first so that apt-get knows
that new versions of packages are available.
dist-upgrade
dist-upgrade in addition to performing the function of upgrade, also
intelligently handles changing dependencies with new versions of
packages; apt-get has a "smart" conflict resolution system, and it
will attempt to upgrade the most important packages at the expense of
less important ones if necessary. The dist-upgrade command may
therefore remove some packages. The /etc/apt/sources.list file
contains a list of locations from which to retrieve desired package
files. See also apt_preferences(5) for a mechanism for overriding the
general settings for individual packages.