You seem to contradict yourself there.

> What attacks are they even mitigating?

The page is based on this email to users@dragonflybsd.org

    Date: Tue, 11 Aug 2015 10:32:58 -0700 (PDT)
    From: Matthew Dillon <dillon@apollo.backplane.com>                        
    To: users@dragonflybsd.org
    Subject: Running firefox a bit more safely - HOWTO

    As people may know a firefox hack was found in the wild that allows
    any accessible file to be read.  This hack was due to a bug in
    firefox's internal PDF viewer.  The exploit that was found in the wild                               
    was trying to snarf developer-related files such as .ssh configuration
    files and keys.

    [...]