Today, when I check for updates in my Gentoo machine, the new Firefox 48 was released. But when I tried to update I kept getting this error:

configure: error: you don't have NSS installed or your version is too old


I tried to enable the NSS USE flag, but did not work, so I went to the Gentoo forums and asked there. After a few minutes they answered me, it is a current bug that need to be solved. But fortunately there is a workaround until the bug is fixed:

echo "=dev-libs/nss-3.24 ~amd64" >> /etc/portage/package.accept_keywords 
emerge -u nss 
emerge =www-client/firefox-48.0

Executing the above in a terminal as root will force portage to install the version 3.24 of NSS, and then firefox will not complain about the old version.

Once firefox 48 is installed, we can enjoy the new multiprocessing feature.

Thanks to fedeliallalinea for solving me the problem in the forums!