Trying to get Adblock to work on local HTML files

I'm trying to detect adblock when I open an HTML file locally on Firefox. So far my solution has been to look at the blocklist - I see that js/ads.js is on the blocklist so my script is

    if(window.myVar === undefined) {

    } else {
        alert('No AdBlock');


Then in my js file I have myVar = false - the issue is that I can never get the file to be blocked by ABP - it always manages to load it. Is there any insight on what I could do to get this to work? Thanks