Extension to stop Firefox stealing keystrokes

I have to use Firefox with a web app that uses Emacs-like keystrokes, including ctrl-w to cut text. My Emacs muscle memory often has me hitting ctrl-w without thinking, whereupon my Firefox tab abruptly vanishes, because ctrl-w is a Firefox shortcut to close a tab. This is maddening. Unfortunately, this occurs on a network where I have no admin privileges, and cannot even get permission to install any software from outside without divine intervention. I’ve found no Firefox setting in about:config that can stop this — I’ve googled and googled, and nothing works.

But it occurs to me that I could write a small Firefox extension that intercepts keystrokes before Firefox proper gets hold of them, and just forwards them to the app. Is this possible? I went through this post successfully. Then I tried to write an event listener for keystrokes, following onCommand, as follows,

document.addEventListener("keydown", function(event) {
    console.log("Keydown:", event);
});

window.addEventListener("load", (event) => {
    console.log("page is fully loaded");

    document.addEventListener('keydown', function(e) {
        console.log('document keydown event');
        console.log('key:', e.key);
        console.log('ctrl:', e.getModifierState('Control'));
    }, false);
});

window.addEventListener('keydown', function(e) {
    console.log('keydown event');
    console.log('key:', e.key);
    console.log('ctrl:', e.getModifierState('Control'));
}, false);

The load event shows up on the console, but nothing else. What am I missing?