!!! All ebuilds that could satisfy "=firefox-60.0::gentoo" have been masked. !!! One of the following masked packages is required to complete your request: - www-client/firefox-60.0::gentoo (masked by: package.mask, invalid: DEPEND: missing whitespace around ')' at '))', token 54) For more information, see the MASKED PACKAGES section in the emerge man page or refer to the Gentoo Handbook.
Hm, I can not reproduce this from ::gentoo repository. You using probably (the current broken) mozconfig-v6.60.eclass from mozilla overlay?
> You using probably (the current broken) mozconfig-v6.60.eclass from mozilla > overlay? yes
Bug is very much valid and has been fixed. commit 320097f70156db4cb8935aaba3d5159649241868 (HEAD -> master, origin/master, origin/HEAD) Author: Jory A. Pratt <anarchy@gentoo.org> Date: Tue May 22 08:43:00 2018 -0500 eclas/mozconfig - Fix missing whitespace around rust/cargo deps