Okay, I've figured it out, thanks to the "test distros online" thingy to test version 145 and 148 as I previously mentioned.[0]

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...