I added =net-www/mozilla-1.7_rc1-r1 ~x86 to my /etc/portage/package.keywords file, and other combinations, like 1.7*, 1.7_rc*, 1.7_rc1*, etc... but emerge seems to ignore this entry.
because it is masked via package.mask. to unmask you need to add =net-www/mozilla-1.7_rc1-r1 to/etc/portage/package.unmask
As Sven said.
Why is it masked via portage.mask? 1.7 is Mozilla's new stable release, like 1.4 was. I don't think 1.7_rc1 or 1.7_rc2 will be that unstable.