Greasy Fork is available in English.
Shows images and videos behind links and thumbnails.
< Σχολιασμός για τον κώδικα Mouseover Popup Image Viewer
It works for me in Mint, so I guess you have another application/extension that intercepts the key or it's a bug in the browser.
Bug? Popup showing on Ctrl doesn't work using Firefox on Debian. Just the automatic or the right click.