I don't think you can send a 403 response back using an iRule in 4.x. This would be pretty straightforward in 9.x. If upgrading to 9.x isn't an option, you might try testing a rule that redirects prefetch requests to a sorry page.
Else, a better option could be to use a robots.txt entry on your site which specifies FasterFox should not prefetch for your domain:
http://fasterfox.mozdev.org/faq.html
+ I'm a webmaster, how can I prevent prefetching?
Because some websites may not have the resources available to support the enhanced prefetching feature, it may be easily blocked by webmasters.
Prior to generating any prefetching requests, Fasterfox checks for a file named "robots.txt" in your site's root directory (subdirectories are not checked). If this file contains the following 2 lines, no prefetching requests will be made to your domain:
User-agent: Fasterfox
Disallow: /
Aaron