In the first screenshot you see third party news stories above the fold and in the second story there's nothing but knowledge cards above the fold. Both are presenting relevant information (and no ads) but which format is preferable? My guess is that certain knowledge cards (like stock price history) are subtly less functional on FF so they are omitted.

I'm seriously skeptical of that. It's plausible they didn't bother trying to get it to work in FF, and just thought "screw that". But judging by de screenshot of the stock price history alone, clearly FF has no trouble rendering that history - both screenshots are FF, right? It's only the UA header that was changed.

Incidentally, this is far from the first time google has been caught heavily optimizing exclusively for their own browsers, which, to be clear, is quite probably not explicit policy - but is monopolistic abuse nonetheless. Remember the youtube polymer redesign? The Edge HW acceleration-breaking invisible layers? I get the feeling they just don't test in other browsers nearly as well.

A static screenshot does not fully capture the functionality of a web element, hence my use of the term "subtly".

Occam's Razor suggests you're looking for ghosts, and just to be sure I tried this on my phone: as far as I can tell behaviour using FF + chrome UA addon (which I specifically installed to test this) is indistinguishable from chrome as far as google search's stock ticker display goes. focusing points, time spans, toggle modes, clicking the extra-info chevron... all no difference.

Given google's history here you really shouldn't be giving them the benefit of the doubt in this matter; at best they're consistently, repeatedly negligent; but it's at least conceivable there's culture of giving additional other google teams; which is a form of monopolistic power abuse.

Oh yeah, that didn't even register to me. But still, when you get recommended stories vs when you get info cards is such black magic that i wouldn't assume that's necessarily firefox vs chrome either.