after upgrading to libevent-2.1.5 and rebuilding dependent packages with cave fix-linkage firefox refused to start as it was trying to load the old libevent.so.5 lib. firefox rebuilt fine and works with libevent-2.1.5 but there is no dependency in the ebuild on libevent so tools such as cave fix-linkage or revdep-rebuild are not detecting the breakage.
All firefox ebuilds inherit mozconfig-v*.eclass, and all of those set an appropriate DEPEND/RDEPEND on dev-libs/libevent.