anti-keygrabber.user.js

// ==UserScript==
// @name        anti key-grabber
// @version     1
// ==/UserScript==

document.addEventListener('keydown', function(e) {
    // only if Command key is pressed
    if (!e.metaKey) {
        return;
    }
    
    switch (e.keyCode) {
        case 188: // , (comma) - open settings
        case 87: // W - close window
        case 84: // T - open tab
        case 76: // L - focus awesome bar
        case 74: // J - open downloads panel
            e.stopImmediatePropagation();
            break;
    }
}, true);