Larger window scroller is fine on Chrome, Edge, but not Firefox

Hi everyone, I just reinstalled the website cheap songbooks dot com and am really happy with the "larger image" window on Chrome and Edge because they give me a scroller window allowing shoppers to view the entire songbook. BUT on firefox I just get a large window. The last time I had the site up, you could click on the full-screen button (top right) and the scroller bar would appear. I wasn't too happy about it, but at least I had a solution to the problem. Now, there's nothing. I found another post that advised going into the includes/modules/pages/product_info and modifying the jscript_main.php file to scroller=yes. I tried that but no luck. Am I in the wrong directory for what I'm trying to accomplish? If you go to the website, just click on any picture on the front page, then click on larger image. Thank you for any help/suggestions.

Have you run the page through html validation to see if there are errors to cause the problem?

I'm not a programmer but I'll give it a shot. One thing I noticed is that on my sister website (cheap sheet music dot com) the large popup works great in Chrome. I must have modified a file but didn't make notes... go figure.