Please bump the dependency version.
I ran into this too (on amd64), =dev-util/cbindgen-0.6.7 is build time dependency.
The bug has been closed via the following commit(s): https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=a8741b1605fc9c1689f2088328d52e7449e0b4df commit a8741b1605fc9c1689f2088328d52e7449e0b4df Author: Thomas Deutschmann <whissi@gentoo.org> AuthorDate: 2019-01-29 08:17:45 +0000 Commit: Thomas Deutschmann <whissi@gentoo.org> CommitDate: 2019-01-29 08:17:45 +0000 www-client/firefox: require >=dev-util/cbindgen-0.6.7 Closes: https://bugs.gentoo.org/676682 Package-Manager: Portage-2.3.58, Repoman-2.3.12 Signed-off-by: Thomas Deutschmann <whissi@gentoo.org> www-client/firefox/firefox-65.0.ebuild | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-)