If you use the Firefox Addon SDK for your extension, you can use the built-in simple storage module . It provides an object storagethat your code can handle as a regular javascript object, but Firefox will keep it persistent for you.
A simplified example looks like this:
var storage = require('simple-storage').storage;
storage.bacon = JSON.stringify({"tasty": "is bacon"});
eat(storage.bacon);
storage.bacon .