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 the var 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;