window.atob() decodes Base64 string, and window.btoa() encodes to Base64 string. These functions are not global functions but DOM Window's methods.

MDC page said as follows:

atob() is also available to XPCOM components implemented in JavaScript, even though window is not the global object in components.

But atob() seems to be implemented in only nsIDOMWindowInternal (i.e. DOM Window.) I could not find for atob() function in any other XPCOM components.

I think a certain DOM Window instance must be required. So I write following code for encode/decode Base64 text from Jetpack-SDK: 

let win = require("window-utils").windowIterator().next());
let {Cc, Ci} = require("chrome");
let atob = win.QueryInterface(Ci.nsIDOMWindowInternal).atob;

return atob(v);

Note that require("window-utils").windowIterator() returns a iterator and calling next() function returns first element (window instance) of the iterator.