To only upgrade Firefox on its own do:

sudo apt-get dist-upgrade firefox

or

sudo apt-get upgrade firefox

from 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.