Created attachment 599998 [details, diff] Fix from upstream commit www-client/firefox-71.0 won't compile with dev-util/cbindgen-0.11.1. More details at upstream bug: https://bugzilla.mozilla.org/show_bug.cgi?id=1602358 Upstream patch works for me.
Assuming no different issue, you are likely building with an outdated repos, this patch already exists in gentoo from commit: https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=7c8c8537773d7a63d54a253dcd21455dbbe48d43 previous bug #702588
A revision bump was done to ensure that this does not happen. Please sync and try again.