Lazy loading is supported in Firefox since version 75: https://caniuse.com/#feat=loading-lazy-attr
I found that this works on Firefox for lazy loading of images. However, YouTube embedding in an iframe, it doesn't work. I ensured that the video is long way below the fold. Even then, the iframe is loaded upfront. This is the HTML for the iframe:
<iframe loading="lazy" src="https://www.youtube.com/embed/iQBat7e0MQs" allowfullscreen="" width="480" height="270"></iframe>
On Chrome, lazy loading works as expected for the same web page (on my localhost).