gPass/firefox_webextension/options.js

var default_preferences = {"pbkdf2_level": 1000,
			   "account_url": "https://gpass-demo.soutade.fr/demo"
			  };

function save() {
    var account_url = document.getElementById('account_url').value;
    var pbkdf2 = document.getElementById('pbkdf2').value;

    browser.storage.local.set({
	"account_url":account_url,
	"pbkdf2_level":pbkdf2,
    })
	.then(function ok() { alert("Saved"); },
	      function err() { alert("Cannot save your preferences");}
	     );
}

function restoreOptions()
{
    document.getElementById('account_url').value = default_preferences['account_url'];
    document.getElementById('pbkdf2').value = default_preferences['pbkdf2_level'];

    browser.storage.local.get().then(
	function(prefs)
	{
	    if (prefs.hasOwnProperty("account_url"))
		document.getElementById('account_url').value = prefs["account_url"];

	    if (prefs.hasOwnProperty("pbkdf2_level"))
		document.getElementById('pbkdf2').value = prefs["pbkdf2_level"];
	}
    );
}

document.getElementById('save').addEventListener("click", save);
document.addEventListener("DOMContentLoaded", restoreOptions);