Created attachment 292733 [details] ebuild that can fetch all locale files I noticed the problem on the ro locale which can't be found by the current ebuild on mozilla's FTP. The fix is to add ${PV} before /linux-i686/xpi in lines 40 and 47 as in the attached ebuild.
!!! Couldn't download 'thunderbird-8.0-vi.xpi'. Aborting.
Thanks for the report, both have been addressed in -r1