GM_setClipboard (and other GM functions) gives an error in a Firefox but not Chrome/Tampermonkey?

Greasemonkey requires explicit @grant statements to use GM_ functions. Whereas Tampermonkey still does some auto detection (a potential security hole).

So:

1. You need to specify // @grant GM_setClipboard in your metadata block.

2. However, this switches the sandbox back on (a good thing), so you also need to make sure you've @required jQuery.

This script will work in both Greasemonkey and Tampermonkey:

// ==UserScript==
// @name     _YOUR_SCRIPT_NAME
// @match    http://YOUR_SERVER.COM/YOUR_PATH/*
// @require  http://ajax.googleapis.com/ajax/libs/jquery/2.1.0/jquery.min.js
// @grant    GM.setClipboard
// ==/UserScript==

document.addEventListener ( "keydown", function (i) {
    var selectLink = $('a').eq (8); // The link by index
    var targetLink = selectLink.text ().trim (); // The link text

    if (i.keyCode === 106  &&  i.shiftKey) // Shift+Num*
    {
        GM.setClipboard (targetLink); // Copy to clipboard
    }
} );