SafeBrowsing works with the binary packages www-client/firefox-bin and www-client/google-chrome, but it does not work if the browsers are recompiled from the source packages www-client/firefox or www-client/chromium SafeBrowsing can be tested on https://testsafebrowsing.appspot.com/ At least, the option should be greyed out to inform the user that the feature is not available. See also: https://forums.gentoo.org/viewtopic-p-8273008.html
www-client/firefox and www-client/chromium are not maintained by the same people. Please file individual bug reports against each package so that the individual issues can be resolved individually.