I tried compiling seamonkey-2.1_beta2 and found that it required some cairo-tee.h header. I still had cairo-1.8* installed and seamonkey errored during the build saying that it couldn't find the cairo-tee.h header. See attached build log. I am still testing whether or not upgrading to the newer cairo will actually fix the problem, but I did verify that cairo-1.8 doesn't have a cairo-tee.h and cairo-1.10.2 does.
Created attachment 263039 [details] seamonkey-2.1_beta2-build.log.gz
No such ebuild exists in portage. The current seamonkey-2.1_beta2 ebuild from the mozilla overlay has a dependency on >=x11-libs/cairo-1.10.0[X] Please reopen if you can reproduce the problem with the latest ebuild from the mozilla overlay.