I’m not logged in, I don’t have an account so there’s no way anything could be cached either. It also does the same thing in a fresh private session. If I switch the page to desktop mode using the regular button in Firefox, I get the following:

I hate to be that guy but it still doesn’t work - same results as my previous screenshots. I reinstalled the extension and restarted the browser but there’s no change. I have been testing this by directly accessing the subreddit via the URL, hand typed into the address bar - not via links from somewhere else on Reddit or the wider internet.

As it’s also not working for me on Android I’ve taken the following screenshots in case they help. I used the same subreddit as your testing, just to be sure.