This is to block firefox from fasterfox prefetching links that the user may never read (this happens on forums for instance quite a lot) and using up bandwidth that is best used elsewhere on your server.
To turn it off add the following to your robots.txt
User-agent: Fasterfox
Disallow: /
Personally I only enable courteous setting on fasterfox, the network tweaks cause problems on some sites and if your broadband is fast enough then it shouldn’t matter. Enabling prefetching will eat into your monthly bandwidth allowance for things you may never read!