Earlier quoted context omitted.
However, I do think you'd be hard pressed to claim that Google does not have the resources to test and make sure that it does work on mobile Firefox.
So now assume they test and find it doesn't work on mobile Firefox. What should they do? Refrain from rolling out the design to Chrome and Safari until Firefox performance catches up, or roll out the functionality differently on different browsers? Sounds like, based on the descriptions here, they're doing the latter, right? Infinite scrolling on Chrome vs. paged viewing on Firefox, for instance.
Fix it, just like any other website out there.