$ emerge --ask www-client/firefox
!!! Problem with sandbox binary. Disabling...


These are the packages that would be merged, in order:

Calculating dependencies... done!

The following USE changes are necessary to proceed:
 (see "package.use" in the portage(5) man page for more details)
# required by www-client/firefox-39.0::gentoo_prefix
# required by www-client/firefox (argument)
>=dev-lang/python-2.7.10:2.7 sqlite
# required by x11-libs/gtk+-2.24.28-r1::gentoo_prefix
# required by www-client/firefox-39.0::gentoo_prefix
# required by www-client/firefox (argument)
>=x11-libs/cairo-1.14.2 X
# required by www-client/firefox-39.0::gentoo_prefix
# required by www-client/firefox (argument)
>=media-libs/libpng-1.6.17 apng

Would you like to add these changes to your config files? [Yes/No] yes

Autounmask changes successfully written.

 * IMPORTANT: config file '/home/kam/gentoo/etc/portage/package.use' needs updating.
 * See the CONFIGURATION FILES section of the emerge
 * man page to learn how to update config files.

emerge: there are no ebuilds to satisfy ">=dev-libs/glib-2.43.4:2".
(dependency required by "dev-util/gtk-update-icon-cache-3.16.4::gentoo_prefix" [ebuild])
(dependency required by "x11-libs/gtk+-2.24.28-r1::gentoo_prefix" [ebuild])
(dependency required by "www-client/firefox-39.0::gentoo_prefix" [ebuild])
(dependency required by "www-client/firefox" [argument])