Gentoo's Bugzilla –
Attachment 316145 Details for
Bug 423313revdep-rebuild does not fix /usr/lib/libstartup-notification-1.so.0 // www-client/firefox-10.0.5 - ld: warning: libxcb-util.so.0, needed by /usr/lib/libstartup-notification-1.so.0, not found