To downgrade a gentoo package you have to tell portage that the latest packages are not wanted. Do this by adding them to /etc/portage/package.mask.
=www-client/mozilla-firefox-3.0.7
That will prevent firefox 3.0.7 being installed when you emerge firefox, resulting in the last previous package being installed unless there are any latter ones, in which case they should be added to package.mask as well.