Interesting. I don't see any real major difference between Chrome and Firefox standard/strict/private in responsive design mode. In strict mode, there is just one ad not appearing, and thus leaving a big empty space where it should be. Otherwise, I still see the "video unavailable" frame, and the page looks basically the same to me.
However, in private mode on an actual phone there is more breakage. I wonder if some of my patches for SmartBlock (which haven't landed yet on Fenix nightlies) may have inadvertently fixed this, since they do impact shims that the page will run. Let's revisit this in Firefox 103/104 to see if the shim fixes did indeed improve the situation here.