Since all the eclass consumers are building from source, virtual/jpeg should be pinned to SLOT 0 to get headers. Filing this bug because I am guessing the change needs to be made in the mozilla overlay to avoid it getting lost at a later date.
+ 28 Aug 2013; Ian Stakenvicius <axs@gentoo.org> mozconfig-3.eclass: + pegged virtual/jpeg at slot 0 in mozconfig-3.eclass + Forgot to mention the bug in the comment, but it's done.