Symlinking the installed files in /usr/lib64/mozilla/ to /usr/lib64/librewolf/ is enough to get it working. I'm not sure adding another entry to the current USE flag mechanism in the ebuild - or keeping the USE flags at all - is all that useful, for the same reason USE=bash-completion is discouraged nowadays. This could probably just install the .py/.json files in /usr/share/ once and symlink the json to all known targets. If someone really wants it disabled for a specific browser there's INSTALL_MASK for that.
www-client/librewolf does not exist in the gentoo repo, and www-plugins/passff-host has no maintainer. You are better off implementing this via an overlay I think.