How to enable CORS on Firefox?

Do nothing to the browser. CORS is supported by default on all modern browsers (and since Firefox 3.5).

The server being accessed by JavaScript has to give the site hosting the HTML document in which the JS is running permission via CORS HTTP response headers.

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security.fileuri.strict_origin_policy is used to give JS in local HTML documents access to your entire hard disk. Don't set it to false as it makes you vulnerable to attacks from downloaded HTML documents (including email attachments).

It's only possible when the server sends this header: Access-Control-Allow-Origin: *

If this is your code then you can set up it like this (PHP):

header('Access-Control-Allow-Origin: *');