Firefox 94, it works. Shortcuts seem to only work in tabs that were opened after you saved the preference. So, if the extension doesn't appear to work, restart the browser and you should be good to go!

I use this to disable Option+Home, which immediately navigates "home" (blank tab) and loses any filled-in form data on the page. Was bumping that key combo all the time because the keys are adjacent on my Kinesis!