I'm using Firefox dev, it overflows for me. Are you sure you didn't hover the earlier screenshot of the unhovered image inside the red square, as opposed to the image inside a codepen embed?
From Firefox 148 release notes for developers:[1]
> The overflow, overflow-x, and overflow-y CSS properties can now be used on replaced elements such as images, in the same way that they are used with other elements. Prior to this, replaced elements were always clipped to their bounding container. (Firefox bug 1999100).
[0]: https://news.ycombinator.com/item?id=49272986 [1]: https://developer.mozilla.org/en-US/docs/Mozilla/Firefox/Rel...