[www-client/firefox] Update for eclass changes.

The firefox-18.0 ebuild has been updated to account for
changes in the inherited eclasses.

The eclass changes caused the python inheritence to show
errors.

The latest ebuilds have a new way of handling jpeg libraries.

Also, the neon section was dropped since it hasn't worked
since version 13.0.

Signed-off-by: Mitch Harder <mitch.harder@sabayonlinux.org>