The lack of Firefox support isn't a big deal because this is a progressive enhancement. Firefox users will still be able to switch icon sizes; they just won't see the fancy transition.
It'd be a shared round trip request for all images (so long as you aren't still using HTTP/1.1) in the 1st example vs a request for the immediate images and then a separate page load.
Both have their upsides/downsides depending on the rest of the page and how users usually use it.
And with the previous implementation, all users would get the progressive enhancement, not just non-Firefox users.
With the previous implementation, all users would get the progressive enhancement… except non-JS users :) https://www.kryogenix.org/code/browser/everyonehasjs.html