Gentoo's Bugzilla –
Attachment 470522 Details for
Bug 616152[mozilla] www-client/firefox-53.0[gtk2] failed: mozbuild.configure.options.InvalidOptionError: 'cairo-gtk2' is not one of 'cairo-windows', 'cairo-gtk3', 'cairo-cocoa', 'cairo-uikit', 'cairo-android', 'cairo-gonk'