The ebuild www-client/epiphany has a running dependency '=www-client/mozilla-firefox-1.5*'. If ~firefox-2.0 is installed epiphany tries to downgrade firefox. the dependency in epiphany should be '>=www-client/mozilla-firefox-1.5*'
Please read http://bugs.gentoo.org/page.cgi?id=fields.html#bug_severity *** This bug has been marked as a duplicate of 149104 ***