Устаревшие пакеты Firefox & Thudrebird

Они замаскированны.
man emerge:

MASKED PACKAGES
       NOTE: Please use caution when using development packages.  Problems and bugs resulting from misusing masked packages  drains  Gentoo
       developer time.  Please be sure you are capable of handling any problems that may ensue.

       Masks  in portage provide three primary functions: they allow a testing period where the packages can be used in live machines; they
       prevent the use of a package when it will fail; and they mask existing packages that are broken  or  could  pose  a  security  risk.
       Masking can be done by two methods: package.mask and KEYWORDS.  Read below to find out how to unmask in either case.  Also note that
       if you give emerge an ebuild, then all forms of masking will be ignored and emerge will attempt to emerge the package.

       package.mask
              The package.mask file primarily blocks the use of packages that cause problems or are known to have issues on different  sys-
              tems.  It resides in /usr/portage/profiles.

       KEYWORDS
              The KEYWORDS variable in an ebuild file is also used for masking a package still in testing.  There are architecture-specific
              keywords for each package that let portage know which systems are compatible with the package.  Packages which compile on  an
              architecture, but have not been proven to be "stable", are masked with a tilde (~) in front of the architecture name.  emerge
              examines the ACCEPT_KEYWORDS environment variable to allow or disallow the emerging of a  package  masked  by  KEYWORDS.   To
              inform emerge that it should build these 'testing' versions of packages, you should update your /etc/portage/package.keywords
              file to list the packages you want the 'testing' version.  See portage(5) for more information.

# ACCEPT_KEYWORDS="~x86" emerge -pv mozilla-firefox
# USE="траляля" ACCEPT_KEYWORDS="~x86" emerge mozilla-firefox

Всем Большое Спасибо!
Разобрался.