I don't know what to tell you. While I stopped using FireFox a while ago (slow IDB blob performance) I just loaded the latest version and it did not error for me. (FireFox 25.0.1)

FireFox claims that it supports the constructor Uint8Array(string) support: see: https://developer.mozilla.org/en-US/docs/Web/API/Uint8Array

so there is no reason why it should be saying invalid arguments. 

Try it with this line instead

var buffer = new Uint8Array(att,data);

But I think that what has to be store is base64, not a string.