See bug 112831 for a crash which is caused by this.
inspector IS building, which is evident by its presence in the linux tarballs. This is purely an installer problem.
inspector.xpi is not present in any -sea tarballs, and not in any linux-xpi directory (so it can't be downloaded by -installer tarballs). In fact, -sea tarballs will fail as described in bug 112831 when you try to do a "Complete" install, or a "Custom" install that includes Inspector. Joe, can you verify which -sea and -installer tarballs you're downloading that include the file?
hewitt checked in the fix for this referencing bug 112831 ... *** This bug has been marked as a duplicate of 112831 ***