Earlier quoted context omitted.
Seems unlikely they'd want to sabotage Firefox. Google is Firefox' primary source of income, and Firefox is strategically crucial token competition that stands between Google and an antitrust lawsuit of the sort Microsoft faced in the early 2000s (where Microsoft were basically a technicality away from being broken up).
It does seem unlikely, but playing with the example UA in curl (as noted in https://news.ycombinator.com/item?id=38925015 ) seems to show it's the combination of "Android" and "Firefox" that causes this behaviour. I wonder if any other mobile browsers are broken?
If they wanted to sabotage Firefox I doubt they would choose to send a blank page. They would probably send infinite captchas or just disable most features (which they already do).