Problem: Apparantly, the Mozilla people saw that the component.reg file was a bit of a problem, and so they did away with it. This causes the unmerging of Mozilla 1.0 to fail, since it tries to regenerate the file with regxpcom (which fails, since it's not there any more) and then tries to chmod it, which fails. I don't see a way to fix it, apart from wrapping regxpcom so that it will touch the regxpcom file for the next few versions. The problem is that the unmerge code is in the stored ebuild in /var/db/pkg, and thus you can't get at it. Personally I fixed it by removing the chmod in that ebuild.
This was only a problem with some versions of portage. It should work fine with 2.0.34, etc as it dont check the return from pkg_*().
