Personally I favor Kaspersky for my ASUS tablet, but I use it (the Windows version) for my laptop as well. It's about $15 a year iirc. I think Amazon suggests if you stay in their walled garden for aps you don't need it (I looked because I thought my mom might be interested, but she likes her dead tree reading format). Personally I'd disagree with that but I'm not a fan of the walled garden approach. And it only takes one outside purchase to drag malware in out of the wild even if their security is as good as they suggest...
*edit* Iirc, the Amazon AP store has AV solutions in it specifically designed to work with the modified Android used by the Fire.