They're both JavaScript 1.7 features. The first one is block-level variables:
let
allows you to declare variables, limiting its scope to the block, statement, or expression on which it is used. This is unlike thevar
keyword, which defines a variable globally, or locally to an entire function regardless of block scope.
The second one is called destructuring:
Destructuring assignment makes it possible to extract data from arrays or objects using a syntax that mirrors the construction of array and object literals.
...
One particularly useful thing you can do with destructuring assignment is to read an entire structure in a single statement, although there are a number of interesting things you can do with them, as shown in the section full of examples that follows.
For those familiar with Python, it's similar to this syntax:
>>> a, (b, c) = (1, (2, 3))
>>> a, b, c
(1, 2, 3)
The first code chunk is shorthand for:
var {Hotkey: Hotkey} = require("sdk/hotkeys");
// Or
var Hotkey = require("sdk/hotkeys").Hotkey;
You can rewrite the second code chunk as:
let Cc = Components.classes;
let Ci = Components.interfaces;
let Cr = Components.results;
let Cu = Components.utils;